My front license plate is bolted directly to the front bumper. No frame is necessary. (Don't really like the frames - blocks too much of the plate, making both the plate and also any writing on the frame difficult to read.)
Depending if a front license plate is not required in your state, your front bumper may not have the holes drilled in the front bumper to mount a plate. If that's the case, I'd suggest:
http://autos.groups.yahoo.com/group/toy ... 008-00.pdf
US TSB PG008-00
May 19, 2000, Front License Plate Installation, 2001 Prius
If you need a frame, you could probably talk to a local dealer and ask if they'll give you one of their promotional ones. Otherwise, the parts department would probably have several available for purchase with a Prius logo and such.