R & B HD in Racine used to have them - I went to the third annual 'slam back in the '70s. I'll post pictures of my R & B 'slam tee shirt one of these days (see here). The 'slams were quite the party - all riders were welcome though the oriental iron riders prudently parked down the street. A running bike would be hauled up to the roof of R & B and one of the guys would rev the crap out of it and send it off the roof to the ground below. Then it was a buck or so for getting to use the sledge hammer on the now downed bike. Beer was consumed.

Oh, I was looking for an R & B reference because I'm pretty sure that's where I bought my old leather solo saddle (it has a 1972 date stamp on the bottom). I may have bought it at the Brit shop that was run by a couple brothers who wanted to install a Nitrous Oxide system on my Norton Commando 850 (purchased from Tommy 'T.C.' Christianson for whom I used to do bike electrics at his shop in Kenosha). Pretty sure I bought the saddle at R & B, though.

About the tee shirts - the Reno shop dropped everything when I came by to get a tire changed on my Electra Glide (HD had a bad batch and a factory callback; I was on vacation and the Reno shop was great at getting me back on the road pronto). Sunset Motors link is above (and, people used to ask me if my name was 'Norton' when I wore that shirt - I finally just started saying 'yeah'). The Triumph shirt is from a great shop in, of all places, Beaverton Oregon.