I've bought a few things from JSR Direct, nowadays eBay mostly. Ocasinally the band's website if they sell any merchandise directly (like my Neurosis shirt which is probably the highest quality shirt I've ever bought from anywhere, had it for years and it's still in great condition despite being washed a zillion times).
Well mostly i get apparel from a local shop called Kelowna Smoke and Rock Shop. But I Also frequent Ebay, and as often as i can i will purchase directly from the band.
I don't buy metal apparel because I'm content just wearing regular clothes. If anyone really cares that much about what type of music I listen to, they can check my facebook.
metalblade, ebay, and wherever the band sells their merchandise.
Either that or I go mega-high tech with a sharpie and a t-shirt.
However, I have has some trouble finding a Kalmah shirt :/
I got some good deals off ebay, buit usual hell's headbangers has some pretty awesome t-shirts. They even have two different shirts for Morbid (The band dead was in before mayhem)
I bought three or four shirts off Amazon, and I mostly use JSR. I got an In Flames shirt and a pretty cool Slayer shirt from Hot Topic quite a while back.
I need to stop buying so many band shirts, I have so many and only wear em around the house on weekends or at shows but I still buy them.
Rockabilia is pretty good but I usually get most of mine from the bands offical stores. Slayer's got some cool classic tour shirts on their store. I've got mostly every shirt from Exodus' store. eBay is nice too if your looking for a real old shirt thats impossible to find anywhere else.
I've gotten them off Relapse Records, JSR Direct, and Ebay, but probbaly Corpse Clothing to get me an Emperor short sleeve in the spring and maybe Plastichead to get some Satyricon wristbands. I balance wearing band shirts with wearing The Simpsons and Super Mario shirts and Dallas Cowboys shirts and jerseys.