I need about 20 cheap, mini cacti for a Southwestern party. Any suggestions of how to obtain?

Check a grocery store that carries Mexican vegetables. They often sell cactus "pads" in the fresh vegetable section, and you could use these for decoration. You could even use these under small bowls of pico de gaillo, as well as using some to eat. I am sure these would be less expensive than buying cactus plants in pots.