Downpipe gasket leak?

Okay, here's the situation. I have a Gibson stainless header and 2.5in exhaust all the way back. I got the header first, then had a shop fab a downpipe several months later. After about a year, I started hearing a leak, which I determined to be from the flange where the pipe connects to the header. I've tightened these as much as I can, and have also tried the dealership and another exhaust shop that claimed to have welded a pinhole up but didn't seem to stop the problem. It almost goes away after tightening, but not completely, then returns over the next couple weeks. I've read on the forum about using brass bolts, which I plan on doing, but I was unaware of the "donut" gasket described in various threads. For all I know I don't have one, but if that's the case, where can I get one, and what exactly is it? I'd planned on replacing the bolts, but if there is some kind of gasket in there, can I re-use it? Also, if vibration is a factor, would lock washers help? I'd considered locktite 272, but am afraid of not getting it right the first time. Any ideas or suggestions are welcome.

TIA!

Donut gasket is stock, with a replaced header you use a different gasket. You can always try a new gasket. The hardest part of the job would be breaking the bolts free, but if you can already tighten them they should be free already.

It's easy for a parts shop to match the old gasket if you bring it with you.