This is a very broad request so far. What characteristics do you want your certificate to have? classical? modern? formal? casual? American? European? feminine? masculine?
I know there have been a few posts about type for wedding materials, such as invitations, but I'm looking for suggestions for type for a wedding certificate.
Some examples are here (these are just examples for the purpose of this post):
http://www.documentsanddesigns.com/galleries/DIRECTORY_Quaker_Wedding_Certificates_Page_1.htm
There's a couple of constraints: script is to be avoided, and sans type is preferred (although serif is ok). Relatively low cost or free type is a plus.
Note that the certificates usually include multiple types, usually one for the body and another for the names or initial lettering.