Where can you find a 5 to 10 round box magazine for a Stevens Model 320 12 gauge shotgun?

I can't find any reference to a Stevens mod 320 however if it is similar to their mod 124 (12 ga, bolt action, 3 shot box magazine, 1950's manufacture) then you will not find an extended magazine for this gun. Stevens never made a 5 shot magazine for this gun and no after market companies would either.

A 12 ga pump with extended tube magazine is a better solution.