As I understand it, the Stampede can take Colt SAA grips, as can Ubertis, USFAs and many other "clones". But Colt grip frames vary somewhat and the various clones vary too (or they're each copied from different era Colts).
So the gripmakers sell oversize grips for all of these that need fitting.
You can either do the fitting yourself or have the gripmaker do it. Personally, I think I'd go talk to Eagle grips as their turnaround time on the gun is likely to be WAY under Beretta's and you'll have a major variety of grip materials to pick from.