I'm in the same situation with my destination wedding in Puerto Rico, and since my fiance is from there, his guys are much more likely to be available. I've been asking my girls one at a time even if I don't think they will make it. The main thing is to let them know that you understand not only that it is a bigger time commitment but also a much bigger financial commitment (unless you are paying for travel/accommodations/dresses which is also an option).
The main thing is to make sure you let them know how much you would love to have them there, but that your friendship will in no way be affected if they have to decline. I've been telling my girls I would prefer them to decline up front, than cancel last minute.
Also, you can always offer to use them in a different role should they want to come but cannot commit to the full role of bridesmaid (like a reader, Eucharistic minister or musician).
I hope this helps at least give you a few ideas. I know this is a tough situation, but please let me know what you decide in case you have any ideas for me as well! Good luck and congratulations on your special day!