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I reckon you should run some t-shirts through cafepress.com or spice.co.uk with a funky retro logo.
Their business model is pretty good for things like this - you just need a design and upload to their site. Direct people to "your" site on cafepress / spice and all the printing, payment and fulfilment is taken care of. You get paid the profit on the t-shirt (no-one will mind you making a couple of quid, especially those of us that know how much of a personal investment it is to run a site).
I've never ordered a t-shirt from one of these site so can't vouch for the quality but have looked into selling on there fairly closely. It looks like cafepress (which is in the US) is cheaper, but the purchaser pays more for the postage, so it ends up working about the same as UK-based spice.
I'd wear a retrobike t-shirt to race / train in. I often get talking to people with older bikes at races and have mentioned this site since I came across it.