The Super 90 M1 is what Benelli introduced to replace the SL121/M121 M1. Consider yourself extremely lucky if you happen to find an orphan stock to fit it. Choate did have some polymer stocks a few years ago but it's unlikely they've made any since then being there's essentially no market for a batch of buttstocks that fit only a shotgun discontinued 30 years ago.
What's wrong with your original buttstock?