Probably simplest to start with something close enough and massage things with a file or die grinder. These are pretty close and you get a pair to work with (for when your projects go like mine and things get messed up)
FORTLUFT Exhaust Flange Connection Kit (Mild Steel, 1.50''/38mm)
Alternatively, you could cut the existing flange off your current exhaust then clean it up and reuse?