Mike these guys have everything plastic. Dealt with them many times.
Here is something you apply to a regular bucket and thus becomes a screw top. Not sure of how sizing goes but I am sure if you call them they will help you out. Pretty much what is said above but you may get a better price from these guys. Maybe not.
http://www.usplastic.com/catalog/default.asp
more specific
http://www.usplastic.com/catalog/pro...oduct_id=20669
These too
http://www.usplastic.com/catalog/pro...oduct_id=20750
Just call them and tell them what you want.I am sure they got it
Threaded Mode