If I ordered a few of them I would get them in a few days here in NY. The gaskets are small and flat and could be inserted into a 'personal' letter to you in Grenada. I have no idea how long the mails would take to get to you though.
Of course it is probably more fun to try and make a suitable gasket yourself. The tolerances of this gasket are fairly important though as you have found out.
Do you know which model Oberdorfer water pump it is? There seem to be at least 2, a 201 and a 202.