I assume you are looking for an adjustable comb and maybe buttplate? I would suggest going over to shotgunworld.com or trapshooters.com and checking the banner ads or searching for gun fitment threads and classifieds. I think I saw a 870 Trap buttstock for sale the other day on trapshooters. Otherwise if you are reasonably handy and have some common wood working tools Midway and Brownells have hardware for adjustable combs and butt plates and recoil reducers like the Graco unit.
As another former racer I am not entirely convinced trap shooting is any cheaper than racing but it is at least a bit safer.
Government is not reason, it is not eloquence, it is force; like fire, a troublesome servant and a fearful master. Never for a moment should it be left to irresponsible action. - George Washington
1911s and V-twin sport bikes make me happy.