Hi Mgrmitch and welcome to MobileRead. The Sony format is .lrf also known as BBeB. However the Sony 505 will also read ePub (the emerging eBook standard) as well as Adobe Digital Editions (ADE). You can convert from one format to another using Calibre which is free and available for download on this site. However, if the book is DRM protected, it won't convert unless you strip off the DRM. There are ways to do that but not on this site.
The Sony site seems to be competitive price-wise. But there are other sites you can buy from such as Fictionwise. Also you can borrow eBooks from libraries using ADE. See the Wiki here
https://wiki.mobileread.com/wiki/EBook_Lending_Libraries for a list of lending libraries. Also lots of info in the Wiki on formats