If you are comfortable with the idea of stripping DRM from purchased ebooks, get the secure Microsoft Reader books and convert them to a format the 505 can use.
First, download, install and activate
MS Reader. This will allow you to purchase and view MS reader ebooks on your PC.
Do a google search for "ConvertLIT GUI". Hint: Use the downconvert option... (can't be more specific here sorry.)
Download and install
Calibre.
Import your downconverted lit file into calibre then hit the convert ebook button. Change conversion settings as desired (font size, margins etc).
With your PRS-505 connected, click "Send to Device" button. Your purchased book is now on your Reader.
Once you have the process set up once, subsequent book purchase will take just a minute or two at most to get on your reader.
In fact, once convertLIT GUI is setup properly, you should be able to right click on the secure lit file in windows explorer and select downconvert without even opening the program.
PS: I recommend trying the free Secure MS Reader book that Fictionwise offers first, to make sure that you get the process working, that way if you can't get it all working, you won't have wasted any money...