I'm guessing you have something similar to the parts in this link.
Kohler Class 6 Parts Diagram
Congratulations on owning a Kohler.
They often build toilets using proprietary parts and as you have found they are often difficult to find. Unless your local Kohler dealer stocks the parts you will be ordering.
Check the curtain valve to make sure the chain is not binding and allows the curtain to fall in place properly. If it is falling in place properly you probably need the #GP1059291 seal kit.
If you do have to order the seal call 1-800-4Kohler for parts. You may have warranty coverage as well but if you can get a $5.30 part local I would.