I've collected mine from all sorts of sources. Some are mine that I have purchased through the years. I have worked part time at a local hobby shop for the last 27 years and people have brought in boxes of them at a time, and we sell them off at 50 cents a copy. I usually got first crack at them, but don't need to any more. There was a time when people used to advertise collections from estates and such in the local newspaper and I have acquired large batches that way. Sometimes you have to take what you don't want or need to get what you do want or need. I usually sell off or give away the extras and I have done that several times over throuigh the years. I financed one of my early trips to VSC by selling off a large chunk of doubles and tripples. Over time, I have built up what some have tolld me is one of the largest and most complete collections in the country. I search eBay every now and then looking for deals of pre-WW Two magazines. Those are the rarest because of the smaller quantities printed and stacks and boxes of them being donated to paper drives. I keep mine in four drawer file cabinets in chronological by title and each mag is in a manila file folder for protection and I think this helps them breathe and avoid smelling musty. These days, I would put a wanted sign up at the local hobby shop and used book stores in you area, watch the weekend newspapers, and maybe put a wanted add in the nearest Craig's List site on line. The AMA occasionally gets an estate donated to them, and they usually sell them off at the Museum. You might try contacting them also.
Good luck with your search and have fun. They are a tom of fun to look through and do research with. In fact, I'm headed to the basement right now to go through some and put some others away back in the file cabinets.
Type at you later,
Dan McEntee