Some info here on various national parks in Thailand:
http://www.thaiforestbooking.com/default-eng.htmAnywhere there is nature in Thailand you can probably a good variety of bugs and plants. Anything in or around the jungle. If there are any waterfalls in your area you might also find some stuff along the trail up to the waterfalls as well.
Khachanaburi is probably only a 2 hour drive and lots of stuff there, waterfalls, etc.