Wellcome to MobileRead, andyj!
One thing you should do, regardless, is go ahead and use the $50 credit at the Connect Store -- just tell the software that you are
in the U.S. when you're installing it. I mean, hey, free books!
If you want to keep using the Connect store, there are a few MobileReaders around here that are willing to purchase gift certificates there and take payment via paypal, etc.
Other than that, since the Reader takes RTF files (among others) you can buy/acquire books anywhere that has them in that format, or in a format that can be converted to it. Baen
(if you like SciFi), Fictionwise.com, manybooks.net, project gutenberg (which I never
spell correctly), etc. can keep you in texts for quite a while.
Hope that helps.