i got mine at a show. i think i remember them posting a bulletin on myspace that all the shirts are sold out, but i could be wrong. i think ebay is your best bet.
i got my god is a failure shirt off of theend records when the cd came out last year so i doubt that really helps you sorry. but it is a place to check!
I've bought all mine through The End Records (longsleeve Mantle, 2 short sleeve PF's and 2 short sleeve Ashes). If you want an official one you just have to get lucky and keep checking The End's website to see if they put any back on sale. Ebay works too.