Here is the perfect tutorial on ball pits.
http://www.sugarglider.com/glidergossip/topic.asp?TOPIC_ID=45571
Because you are out and about now, I'll name some items suggested so you can pick them up.
Wiffle balls, golf balls, ball pit balls.
Easter eggs, all sizes.
Toys s/a: rubber duckies, rubber animals, plastic army men, plastic dinosaurs, plastic animals, plastic boats, plastic airplanes, large plastic jacks, treasure chests, and the like (check for decent quality, not rough edges).
Plastic bracelets & rings.
Good quality and medium to large size pom poms, (avoid tiny & small ones, and pom poms that fall apart easily).
Shop in party, pet, and children's toy sections of stores, party supply stores, craft stores, and my personal favorite, the Dollar stores, especially Dollar Tree.