There's almost no reason to ever pay for a PDF manual for the "black" singers. The people selling them, unless they're printing and mailing them to you are a rip off. They're all available on Singer's site, or ISMACS. The originals of course are a different story.
This is assuming Owner's manuals are what you're looking for.
Service manuals are a little harder to find, but the first one I would suggest there would be the WeFixIt Yahoo Group. That's where my 201 and 301 service manuals came from for example. Singer has the 306, 401, etc service manual on their site...