Where can I get document signing certificate

Question

It seems like everywhere I've looked they are personal certificates on a usb key and they are used to prove personal identity. I have a website that produces documents in PDF. I would like to sign them so that they are credible in a way that they were issued by my web site and haven't been tampered with...

Doing this with self created certficiate works for me, but I would go live with that only if it was my last option, so - where can I purchase certificate for document signing?

EDIT - to all who answer to this question with GENERIC "any certificate authority will do" please be MORE specific. They (Goddady, Verisign, Entrust, etc) sell certificates for mail and document security on USB devices which will not do for me.

Answer 1

These are the Adobe Approved Trust List members:

http://helpx.adobe.com/acrobat/kb/approved-trust-list1.html

And this is the list of partners Adobe mentions on their website for Adobe Certified Document Services:

http://helpx.adobe.com/acrobat/kb/certified-document-services.html

Answer 2

Entrust, Verisign, and Thawte all provide digital signatures. Thawte is even free!

Hope that helps!