I'd suggest a place like Numrich Gun Parts
Magazine, 12 Ga., 2 Round
I found that mag in the model 58 section they also have parts for the model 58 series F, and 58A, 58AB, 58B, 58C, 58D, 58DAC
these may just be cosmetic diffrences but just make shure if its a straight 58 or a Letter designation, Maybe they had a diffrent way the mag latch worked?
I bought my shotgun through J&G Sales in Prescott AZ, 20ga Stevens 258A for sale $39 missing mag and chipped butt plate, I bought a new mag and buttplate and My kid uses it for Ptarmigan hunting here in Alaska
Note about my replacement 2 round mag.
The 2 round replacement mag I bought for My Stevens 258A 20 ga ($30) was stated as a 2 round mag but no matter how much I try I can only stuff one round in it, the second only fits 3/4 the way into the mag with the follower and spring totally compressed flat, so basicly I have a Bolt action mag fed single shot, or one in chamber one in mag, I dont really mind it all that much the mag only holds a single round, Just wish they'd a said so to begin with, Hope the 12Ga is diffrent.