Do you mean people shirts with horse stuff on them? I have ordered from Cafepress http://www.cafepress.com/ and Country supply It think Cafepress has the best sellection. You can get your Horseforum.com bumpersticker through them too
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France
My fiance and I have a store where we can press shirts. I made an "appaloosas a spot above the rest" with a picture of an app between appaloosas and a spot above the rest. I think it came out really well.
I am not sure what you need but there are LOTs of places to find horse tee shirts.... if you mean tee shirts for people with horses on them...
I did a quick search and put a list of what I found on this page:
I hope it helps you. I will leave the page active as long as it is getting attention. Have fun. Sorry, I did not have a template to make all of the links active , so you will have to cut and paste and put them in your browser. I am sure there are more out there.