I don’t know of any source for good looking, plastic cactus – or especially desert shrubs – in a suitable scale. I made all my stuff, mostly by modifying various plastic flowers or greenery from Michael’s. Unfortunately they stopped selling the one I used the most, but I keep hoping they’ll bring it back eventually, or something like it.
Here’s a link to how I made my plants. Some of the info is out of date, though. For instance, I now use floral wire instead of pipe cleaners, for the ocotillos:
http://www.raydunakin.com/Site/IRR_Plants.html
Of course the kinds of cactus and shrubs you need will depend on the type of desert you are modeling.