I have found solved the problem a couple different ways.
First, there are donuts that will fit. Typically they are the more expensive ones at the part store (they usually offer 2-3 brands at NAPA or Shucks/Checker/Kragen). The better quality seem to fit better on the Magnaflow pipes. I have had good luck with the Felpro gasket, but lately they have even been substituting the low quality ones and rebranding them as Felpro.
When I can't get my hands on the right donut, I have made a low cost one work by using a dremel/die grinder/rat tail file, to gradually open up the inside. It is a pain, but it will work.
FYI, this is one of my personal parts problem pet peaves, so we at SHO Source are looking at identifying and stocking the correct fitting donuts so you can buy them in the future and know you will get the right part.
Mike