Code Signing Certs
The certificate you will need to use for signing PAD files is the same certificate you use for application code signing.
Locating a good provider
The following sites are known to have fine code signing certificates:
Choosing the right type of code signing certificate
PAD signatures follow the XML Signature Syntax and Processing (Second Edition) Standard, which allows for a number of different code signing certificates.
How you choose really depends from here on which third party PAD signing tool you choose and what it supports. See that vendor's website for further details.
Unfortunately, the author of Padded cannot provide technical support for these third party tools.