Kelley Blue Book
2004 was the first year of the gen 2 body style, if you go with a first gen Prius you can get one for $3,000 or less, go with a 2004 and the blue book value ranges from $4,300 to $5,300, well within your budget.
Look for poor gas mileage, brake rotors that are rusty from lack of use and of course other things you'll find on any 10 year old car.
Test drive a bunch of them before buying one, you might even start out at a dealer looking at used just to get a feel for what they should drive like if everything is working right.