Chargrilled.co.uk are an excellent T-shirt company who sell a big load of T-shirts. There are some really excellent designs to choose from (says Emma while sitting in her Jimmy Saville T-shirt). I've bought from these guys a few times, and I highly recommend the service.
Their offer at the minute - they will give you a fiver off if you purchase any 3 items - just add them to your basket, and you'll see the fiver discount at the checkout. Now all the t-shirts are around a tenner... so you might be spending a bit in this shop, if you like the designs!
Quidco.com also offer a WHOPPING 20% cashback as well.