This topic came up on a recent Acrobat session, raised by people discussing cutting and pasting signatures into documents to simulate signing. Which led to a discussion on digital signatures and Acrobat and a run through of the process (using a self signed certificate).
So I thought I would dig up some light reading on how to get a proper digital certificate for document signing.
An Adobe article that links to approved providers, (note that Verisign is now Symantec) leads with a little digging to the following resources.
Document Signing Certificate providers
Which should be enough reading to get people started.