You could check your local Auto Zone or Pep Boys to see if any bulb manufacturers have have made a "beeper bulb" of the bulb# used in your Prius's back-up light. A couple of years ago when I last checked there were only 2 such bulb #s in a "beeper" version & neither one was the Prius back-up bulb #. If there is one for the Prius now, I would recommend getting 2 of them because with the bulbs being inside the sealed tail light unit, I'm not sure if the sound level emitted would be loud enough with only one bulb [I imagine you would hear them better inside your car & that may become very annoying---just look here at all the people who have had their internal back-up beepers (the "you shifted into "D" too fast, but you're still in reverse so don't punch it yet, dummy") disabled by their dealers].
There is an item that was in/on J.C. Whitney's catalog/website that might be more practical. It was a license plate frame that had a 2-tone beeper & a strobe light (additional safety measure at night). That might be a better choice.
That's the tricky part & why I haven't bought & installed the license plate frame strobe & beeper since I discovered it about 2 years ago. I would really like to do it, but I'm not very good at car work any more (I used to be many years ago, but I'm now age 61 & my patience & dexterity suffer especially with my arthritis). Wiring the license plate frame would involve running a wire into the hatch & threading it up along whichever side the cable harness is (for the license plate lights, rear wiper, & electric hatch lock/unlock), then out of the hatch & into the car body near the roof & back down to one of the tail light units to splice/tap onto a back-up light wire (they have little kits for safely splicing/tapping onto a wire that have protective insulation too), but the real problem is getting the plastic trim panels off without cracking one of them (safer to do in the summer when plastic isn't as brittle) or breaking or losing any of the trim clips (not sure if replacements are easily available through your dealer's parts dept).
My real preference is to do the above, but because of the relative complexity I have put it off for the last couple of years, however I might do 2 of the back-up beeper bulbs (if anyone now makes one for the bulb # the Prius uses) for the simplicity of installation [as long as the sound level inside the car isn't too annoying---some foam insulation might help deaden the sound as long as the backs of the tail light units don't get too hot (tail light & back-up light are regular "hot" incandescent bulbs & not the cooler LED's of the brake lights)]. If you go the "beeper bulb" route (rather than the license plate frame), let me/us know the bulb # of the one that works in the Prius. Thanks.
There were 2 other alternatives in addition to the 2 "clean & dry" ones above. I found 2 other back-up warning devices locally that were rather large (about the size of a baseball, if I remember right) that would have to be mounted under the car near the bumper (an easier wiring hack), but would be exposed to the elements on the road (water, snow/slush, mud etc.) & probably wouldn't last very long. The auto parts store let me sample the sound of them by temporarily hooking up a 12V battery. The simple "beeper" one was loud enough to do a good job, but the other one had an electrically generated verbal warning stating "This vehicle is about to back up" which kept repeating & was really rather annoying, & again both of these would probably have to be mounted under the car (where there is already very low ground clearance) & be exposed to the elements (more so than with the license plate frame).
My reason for looking into this back a couple of years ago is that a fellow Prius owner in the next town to me told me that he was being sued for bumping a pedestrian behind his Prius. His story was that another car "zipped" into the parking space next to his & the woman driver quickly jumped out & briskly walked behind his car (which had been on already & had gone through the warm-up cycle & shut down the ICE already). He had seen the woman go behind his car in one of the outside rearview mirrors, but didn't see her any longer in the inside rear mirror or the other outside rearview mirror & since she had been in such a rush, he just assumed she had cleared out of the area behind his car & was probably behind other cars now on her way into the store(s). What he didn't know was that she had dropped her keys behind his car & bent down to pick them up & put them into her purse. He put his silent Prius into reverse & felt a slight bump & heard a loud scream of fright. He immediately got out & found the woman on the ground in shock that a car with no engine running had just knocked her down & might have run her over. She had no visible signs of any injuries & immediately got up & said she was fine. The Prius driver offered to call the paramedics to check her out, but she refused any medical attention & went on her way. About a month later the Prius owner got a letter in the mail telling him he was being sued for many thousands of dollars (about $300,000 I think he said) by the woman he bumped. It seems the woman's son was a big shot lawyer in New York city & he viewed the Prius owner as a "cash cow" for him & his mother.
I never did see that Prius owner again, so I don't know what the outcome was of the suit against him, but it did get me to start thinking about an exterior warning sound for a silent Prius in reverse. His Prius was a 2nd gen (2004-2009) & the only generation I can vouch for (mine is a 2005) where the back-up beeper we hear is only internal to remind us of the fact that we may still be in reverse (if we flicked the joystick shifter too quickly into "D" & it didn't engage) before we step on the go pedal & have a crash. Many new Prius owners don't know this & assume the beeper sound is heard outside as well, but it's not so unless you do a modification on your own.