Before Ruger started to sell 20rd and 30rd magazine to the public, I only purchased Fed Ord and PMI magazines, which fit and functioned in my Mini-14s... Sometime around 1996, I picked up a few “unknown” brand of Mini-14 30rd magazines at a gun show for what was a good during that time, but a total waste since they barely functioned and had locking issues as well. Learned my lesson and stuck with Fed Ord and PMI magazines only.
One time, around 2002, I went to an old “Fudd” gun shop up in Alaska and asked if he had any boxes I could dig through to see what catches my eyes. He brought out four or five bins and I found four PMI 30rd mags, still sealed in their beat up bags for $25 apiece... I also found two S&W Model 39 magazines.
A side story, when to another gun shop a few years later and saw that their magazine bin had two Heckler and Koch HK-93 old school, and slightly rare, 20rd alloy magazines... I have to conceal my excitement since he was watching me for my expressions, so I played it off like I was getting ready to put them back in the bin. With my best poker face causally asked the price and he said $25 each or $40 for both, if I buy them right now. Gave him two Jefferson’s and left... Couldn’t believe it since they were going anywhere from $80 to $100 back around 2007.
About six months ago, here in Washington State, I did the same thing again and found two Ruger Factory pre-ban 20rd magazines for $20 each.
OP, I’d strongly suggest you check out some of the old timers gun shops that are off the beaten path in the New England area and ask to see their spare magazine bins... I’d bet you’ll find either Fed Ord or PMI and possibly Ruger pre-ban magazines.
A good sign that you are where you need to be is that if the shop stinks of cheap cigars and the stink is on everything... You will find what you’re looking for