There's a little trick if you have an american express card : I live in France and I was able to register my PRS-505 with my american express card, but because the card number is somehow attached to my adress in France, I had to find and tell I live in an US city with the same zipcode as mine (thanks google). Fortunately zipcodes from USA and France share the same 5 number format...
On the other hand there's a good site from where you can buy BBeB ebooks : fictionwise