There's the Silverado someone said was "the best bang for your buck" a while ago, but we've also heard from people who have got tiny burns from it. I guess it has crystals and coils etc. so if they work it might be good. But seriously if you want a high-powered model I suggest ParaZapper, which is a reasonably priced and well-supported specialty zapper.
If you want the "cheap mileage-maker" with no frills, the zapperplans.com $10 circuit is a good choice. Just don't be alarmed if it's so small it looks like there's nothing there; it's really a good working miniature unit under the black heatshrink casing.
Whichever model you get, take it slow to start with, drink plenty of water etc. (some people get nasty reactions from all the suddenly dead parasites in their bodies.) Good luck.