The metal keytag that comes with your keys at purchase is the key cut code for the metal key. If you loose the last metal key, a new key can be cut from the keycode on the metal tag without the need to copy a physical key. HOWEVER, this tag code has nothing to do with the engine immobilizer security code! The metal code on the keytag will only get you a key to open the door, but not start your car.
The Prius has an engine immobilizer, so it must recognize a transponder chip in the key as one it knows about before it'll let you start your car. To program a new key into the car's security ECU (one of the computers), you first need a recognized master transponder chipped key. (For a Classic Prius, this is usually a black master key (NOT the grey valet/subkey). For the 2004+ Prius, any keyfob is a master if it starts your car.) DO NOT LOOSE YOUR LAST MASTER KEY! The security ECU cannot start the registration process for a new key without an already existing recognized key. If you loose all keys, you'll have to replace the security ECU with a fresh one, about US$1000. (a few years ago I weighed my Classic Prius' black master key, and figured using the spot price of gold for that day what the black key would be worth if it was made of gold. The replacement cost of the ECU is more than the price of that key made of pure gold, so that last key is literally worth its weight in gold!)
For a Classic Prius, you can purchase the keys online (like eBay), or here for $18+shipping: http://www.coastaletech.com/carkeys.htm
You can have the metal key cut at any locksmith/hardware store.
You can program a Classic Prius yourself, although it may take a few tries to get the timing right: http://www.coastaletech.com/keys.htm
You can have 4 master keys and 2 sub keys.
(A master key is one that can start the reprogramming process of a new key. A sub key is allowed to start the car (disable the engine immobilizer), but it cannot make a new key. The grey key that comes with delivery is usually a sub key, and also has a different metal keycut to make it a valet key (cannot open the trunk).)
For a 2004+ Prius, note that there are two types of keyfobs available, a silver Toyota logo for a Prius with SE&SS/SKS, and a black Toyota logo one for a regular non-SE&SS/SKS equipped Prius. I'll note that on page 18 of my 2004 owner's manual it plainly states:
"Your Toyota dealer will need
one of them to make a new key with
a built.in transponder chip."
Shop around for pricing for the correct keyfob, discounts can be found at different dealers.
Although there are programming instructions around for doing it yourself, the instructions came from a German manual and may only work in that country (hasn't worked on a US Prius yet...) You will have to go to your dealer and have them program it for you using their special tester that talks with the computer. Be aware that you'll probably need to take all existing keys in to have them all recorded with the car's computer. Again, shop around for a dealer that has a lower per-hour labor charge, and one that has done this before (so it doesn't take forever while they learn the timing with your vehicle), as they have to program not only the car to recognize the keyfob, but also program the remote entry buttons (and also the SE&SS/SKS if so equipped).
Again, you can have the metal key cut at any locksmith/hardware store, for probably less than the dealer...