Sorry, I've never looked at the Sony store, I assume they don't sell mobi formats? Didn't look much at Amazon either after I found out my money wasn't good enough for them
There are sites for searching for e-books and comparing prices. I've looked at a couple that I found from posts here, but I'm not really using them (sorry, can't remember the names). They might become useful though, now that the number of sites selling e-books is increasing.