20rd M4 mags themselves are pretty much a dead item; I've not seen anyone legit using one of things in years (outside of bench shooting or ban states), and as you've noticed, the market reflects that. 20rd mag pouches were always a niche item, they are pretty much discontinued everywhere now.
Still a couple companies that make open-top pouches (bungee, elastic, kydex) that are either 20rd specific, or short enough to fit 20s (like "quick draw" type), but flappers? Yeah, nah. Deader than Kennedy... 
There are probably still some pouches you could press into service - I used to have several of the TT fitelite smoke pouches, and when adjusted full out, they could just fit a 30rd, so I'm pretty sure they'd fit a 20 nicely. YMMV
TT also used to make some 20rd bungee shingles, but they don't retail them anymore - you can special order them in quantity of 20+.
https://www.tacticaltailor.com/fight...mokepouch.aspx
You could try to search for old-gen milsurp. IIRC there used to be some decent British 20rd flapped pouches floating around. Unless I'm getting them confused with something else....
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