There are lots of possible sites, but
Books on Board in the US and
WHSmith eBooks in the UK are good starting points. Buy Adobe ePub (1st choice) or Adobe PDF (perhaps called Adobe Digital Editions). These ebooks will work "as is" on your PRS-505, although PDF isn't a good ebook format and the PDFs will have some reflow issues.
If you are willing to strip DRM (encryption) you can buy other ebook formats, and that may allow a slightly wider selection of ebooks, but Adobe has a wide selection if you don't mind the PDFs.
A 3rd book store to look at is FictionWise, and here I am particularly thinking of their DRM-free multi-format ebooks (which include LRF for the PRS-505). These include a lot of romantic/vampire plots, and they are not always available from other sources.