I just received my ASR muzzle brake from Silencerco for my 450 Bushmaster.  The 450 Bushmaster uses a rather odd thread pitch 11/16x24.  The ASR brake threads on just fine, but the shims that they provided in order to time the brake are slightly too small in terms of the inside diameter.  The shim package says "5/8 shims," so it makes sense that they might be ever so slightly too small.

Any thoughts as to where I could find a shim kit that would work?

I'd email them and ask if they have any that will fit.  I needed some shims last week and couldn't find them on their website so I sent them a message.  They dropped a couple in an envelope for me.

Take inside and outside dimensions that you want and order appropriate shims from McMaster Carr.