Using your printer, you can make your own T Shirt transfer and then iron them on. My two favorites are "FUTURE COmpost" with a steaming pile below that with a pitchfork stuck in the side and hat on top; and my Trowel and Error Gardener shirt. Whatever you can imagine, you can print.
There are many online shop offering logo t-shirts with whatever logo you wanna print or embroider. And you can design the logo by yourself or the shop designer. Here is a nice shop I wanna recommend to all of you: http://www.buycorporateapparel.com/