If you are looking for apparel that both expresses your politics and supports folks with (at least similar) ideals, the first option would be to look locally - many info shops and anarchist book stores also sell a selection of shirts. If you are unsure if anything of the sort exists near you, try checking out the radical contact list maintained by the Slingshot folks: http://slingshot.tao.ca/rclist.html.
Any amount of time with this list will tell you that not all places have physical anarchist spaces that might sell t-shirts. In that case, going online might be an option. human mentioned AK Press, who do have a wide selection of shirts, as well as a lot of books, some of them anarchist. Little Black Cart and Black Powder Press sell (some) shirts, hoodies and other stuff in addition to having really worthwhile selections of reading material.
Sometimes local anarchists produce local anarchist shirts or other clothes ("Santa Cruz Anarchy", "Oakland @'s", "Seattle Anarchists: Social War 206..." others, St Louis has a good one but I can't recall what it is), or various prisoner support groups or other projects will produce shirts whose proceeds go towards furthering anarchist projects or solidarity/support.
lawrence and human also both comment about making your own. I've never learned to silkscreen well, but it isn't that hard, and you can produce whatever damn message you want if you do learn.
Lastly, most importantly, I have a collection of "anarchist" t-shirts. I like to wear them, and I still pick them up sometimes, but what is your goal in doing so? Are you seeking to find comrades and accomplices? If so, these might be helpful, but not nearly as helpful as actually spending time with people and talking to them, sounding them out to see if you feel affinity with them, and if you have any shared goals. When I was younger and trying to find people, I had the most success at punk shows, but that was a different time, and even then, I found people I liked, many many fewer whose political perspectives held any real interest to me.
Alternately, maybe you want to broadcast your beliefs to those around you. That is totally a thing, and sometimes I like to wear particualr shirts because I want people to know what I want them to know, and I both hate holding signs, and holding both my middle fingers aloft for extended periods. Sometimes it feels good to wear something that places you in opposition to most of the bullshit we deal with day-to-day.
On the other hand, there is something to be said for the ability to remain invisible, or the ability to blend in. I have anarchy t-shirts, but in a lot of my day to day life, while I most often still maintain subtle signifiers that will be messages to insiders, I also wear clothes that allow me to pass through spaces or access things I might not be able to if I was wearing a shirt with a burning cop car and a circle-a on it. There is something to be said for just plain black.
