You should be able to get yellow jacket traps at any hardware store. The kind I use are basically just an ugly transluscent plastic cylinder with screw tops and bottoms with little holes the yellow jackets can crawl into but not out of. You bait them with a hunk of decaying meat, and they crawl in and die.
They actually work quite well. Hang them up from tree branches about 30 feet apart. They sell special bait, but I've always found that a wee chunk of hamburger works fine -- after two hours in the hot sun it's plenty rank enough to attract any yellow jacket within smelling distance.
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