What's the best program to volunteer in Thailand through?

[Charles]

I guess the best way to volunteer is directly to an agency or preserve park or the like instead of being filtered thru another layer, but do not know for sure.

Also, by "good price" do you mean to pay an agency to be able to volunteer there or do you expect to be paid to "volunteer"????

In any case, any kind of work, very broadly construed, in Thailand by a foreigner must be done with a work permit, volunteer or paid. The agency should protect itself from engaging undocumented workers/volunteers and also must cooperate in getting the work permit, so this is not a drop-in activity.