I buy UK ebooks all the time with a US credit card, and they seem to be happy to take international orders. I recommend
WHSmith, where you want Adobe ePub (best) or Adobe PDF.
Most US ebook stores will accept international credit cards but you have to work around geographic restrictions on ebooks.
Books on Board is probably the single best of these for PRS-700 ebooks (buy Adobe ePub, or Adobe Digital Editions which are usually PDFs). The trick is to set your country of residence to US, buy a gift certificate with your credit card and then use the gift certificate to pay for the ebooks.