5/16-18x1 header bolts...where to buy?

Having trouble locating a set of 5/16-18x1 header bolts for my truck. Before you say just get 3/4 I can't. The flanges on my headers are almost 1/2 thick so it wouldn't leave me enough threads to tighten. Any help would be great, I perfer stainless but I guess beggers can't be chosers huh?