I wish I knew where to get a T-shirt like that.
It will require a bit of leg work (so to speak).
First, you can copy the pic here from the site (observing all copyright laws and such).
2. Go
to http://vectormagic.com/ and follow the directions. It will send you back a vectorized art file. I haven't done it in a while so its a bit foggy, but you will want it in a .EPS file.
3. Take the .EPS file to your local T-shirt joint and ask them to print it for you. You may get a little greif for the similarity to Snoopy, but if they are not for resale, it shouldn't be a problem. White T shirts will be cheaper and save having to print the white in the design. It looks to be 3 or 4 colors. Set up is the big expense especially for just one. If you wish, make a bunch of buddies a Christmas/birthday gift. Each shirt should be between $3-5 plus some set up fees for the screens. If you spend about $75 - $100 you should have a good amount of shirts for distribution. Again, selling them with the Snoopy graphic, although changed a bit from the original, could cause problems, but gifts should be good to go.