Looking around in RP for used quads can be tough to find, stay away from the rentals you see around town.
You (or a friend) could put a wanted ad on the board in JJ's for some, there are plenty sitting in peoples garages (in Cholla and other places) not being used since Rhinos/Razers have taken over.
If you don't mind buying Chinese brands, the Electra store in RP sells new quads for about $1,200. (you can buy some brands in the US but more $$).
Most of the 2 wheel bikes and scooters you see driving around RP are Chinese, my local friends have 3 of the 2 wheel bikes and love them
Mechanically no problems in a little over 1 year. For just putting around they can make sense.
Buying in the US is easy, plenty of options to choose from, only small issue is getting them across the border, having the Mexican customs
people look them over and check S/N's, just need titles for any you bring down.
Here is the link for Quad hunter,
http://www.quadhunter.com
Good luck.