Are you looking for specific art or just any old art? If it’s the latter, there are several repositories of public domain images out there. I don’t know enough about it to suggest one in particular, but this might be a good start. If you are looking at specific art to put on a shirt, then you’ll first have to determine its copyright status. Any copyrighted work you see should have a copyright notice attached to it (but if you don’t see this, it does not mean that the work is not in the public domain!) The notice should have at least a name and ideally some contact information. You can use that information to contact the copyright holder in order to negotiate a license for use.