try jakesdoghouse.com, they've got all sorts of sports apparel, collars, leads, tags. etc. the collars are a little big for a maltese, but the buttercup has a great baltimore oriole tag and shirt from there
ann marie and the "leave my raffy alone!" buttercup
www.sportyk9.com makes some great authentic NBA, and MLBPA stuff, really awesome.
I don't think anyone has authentic NFL stuff right now? I know the company we carry that sells authorized college gear lost their NFL License. I think there are some things in the work though from a few different companies, or you can always have someone make you something custom??