The reason that no one imports board bags to the US is that board bags are considered "textiles" and a different import license is required for textiles plus there are other import technicalities that make it difficult to bring them in.
You might try contacting the Amara Windsurfing Center in Pattaya, Thailand.
Amara has good relationships with Starboard HQ so she may be able to get the bags you need and ship them to you.
You might also try Bluefinz in Singapore.
I'll ask Starboard HQ who imports the bags that's closer to the US and get back with you.
Hope this helps,