Like Euroboy is pointing out, I also use my norwegian Visa with the US Sony ebook store. Just find a valid zipcode in any of the States (i.e. google any company and look for its contact address). For the rest of the address, use your own. Country must be USA though.
As for the original question about finding ebooks we have a similar problem here in Norway with almost no ebook stores. I have found great help using this site:
http://sonyreader.no/
[I may have seen it referenced in another thread...]
Even though it is written in Norwegian you will easily find your way down to all the hyperlinks to different ebook-sources in many countries.
ArcticBoy 