Hello Gordon, and welcome
The 505 can read ePUBs, so BooksOnBoard is one possibility, as is Fictionwise. Coolerbooks should help too, as soon as they'll debug their website.
Up there on the left you can see the Wiki, which is packed with great info and lists, there is one for online shops too if you want to have a wider choice.
I enjoy BOB's pricing system (BooksOnBoard), but their search function is not that great...