I have a suggestion...
Every year I go to the States for a Warbird holiday, I normally print off a few designs of my own and iron them on plain old Tshirts. This costs me about £10.00 for the A4 size iron on sheets (normally around 10 to 15 sheets a pack) and you can get specific iron-ons for both white and coloured shirts. These iron-on sheets are readily available in most countries and are probably available in other sizes apart from A4. And all you need is a computer...duh !...and a standard colour printer !
My suggestion is that with all the artisitic dudes on this forum, that we create a 'design depository' where designs that are cleared by those in power are offered for download and then its up to the downloader to get the design he / she likes and iron it on thier own shirts.
Plus marks are no delay in getting the chosen design...and the designs could / would be updated regularly...and the only cost is the iron on transfer sheets. When it starts getting tatty, you have an old t-shirt for whatever, and you simply look at the download pics on offer and do another one. And perhaps more importantly, no-one gets stung with the cost of the original order from a manufacturer and all the associated hassle of getting everyones payment by what ever method, so no-one ends up out of pocket.
Phoooweeee...who wants iron-ons, they dont last !. Well actually, yes they do, some of mine are now 3 years old plus and admitidly the colours are fading a bit, but for what it cost me, I'm not complaining, and if I really needed a new looking one, I'd just print off and get ironing again.
And you can do your signature block with the left overs and iron that on the front as well...great for forum meets !
Another option could be that instead of a site T-Shirt, how about a baseball hat ?. I have one from another forum I belong to and even tho's its embroidered, it was still cheap....you have that added bonus that normally with nowadays, one size fits all with that adjustable band at the back.
I'll do one ...T-shirt !...this weekend and post up some pics for you guys to look at. And dont worry, it wont be worn in public so no worries about copyright etc as I'll use my own pics. this will be purely for a 'test'