You don't have to buy specifically Buzz (or Woody) stuff. Buy the right color paper plates. Buy a plain white paper tablecloth. Print pictures of Buzz (or Woody) from the net and use them to decorate the tablecloth. Use a Buzz doll (if you have one) as a centerpiece or make a rocket ship out of cardboard. A rocketship cake would be easier than Buzz (for Woody, a round cake with a sherrif's star wouldn't be hard).
Print more pictures from the internet and use them to make a banner or mobile or something to hang from the ceiling. Add a few correct colored balloons (you may not need them once you get done hanging Buzz and rocketships from the ceiling) and voila - a party (or as much as five year olds care about).
For games - for my five year old son's I discovered two games worked pretty well (they didn't sit for much more than that). Since its warm, take them outside. Make a big poster of Zurg and give them squirt guns. Have them "blast" Zurg. Also a obsticle course would be a cute "Buzz" theme. Woody can go on a rescue mission to save the "critters" Have them follow a map as fast as they can (around the house, across the pond (your kiddie pool with some step stools in it), save a stuffed animal and return up and over the slide.
Favors don't have to be much (the other mom's will appreciate it). Buzz fruit snacks and some candy in a bag.