I have a few, but no sports related ones. There's a club for people who only collect poster stamps. Some of them are a bit expensive even if most aren't.
There was a Duke Kahanamoku stamp advertising some exhibition on Ebay recently that was in pretty bad shape a big tear on the left side. I was amazed what it went for.
http://www.ebay.com/itm/1914-Poster-...p2047675.l2557
Steve B