I'll provide my data points on magazines. This post is a bit wordy but wanted to provide complete data. I have both the 18" and 24" RFB's and multiple copies of several different brands and types of mags. Some mags work better than others, and I bought extras back when there was more concern of mags over 10 rounds being banned and no longer for sale.
Summary: Most RFB failure to feed issues are magazine related. DSA is a reasonable souce of steel mags ($25 on their website). I've heard they are Kel-Tec's supplier, but can't confirm. I did see at least one post where a Kel-Tec employee suggested trying them as a source, but saw a similar comment for Thermold (hope I don't get anyone at Kel-Tec in trouble). Thermolds work great, don't rust (except spring), are lighter, a great alternative to steel, and are less expensive ($17). Surplus, used, steel mags usually work smoother than new and don't damage brass as much, as long as they are not dented or rusty inside. However, buying used is hit or miss. Most used or surplus sellers will embellish the condition of their product. I've bought several supposed "new" surplus, and none were flawless. Some did work very well.
The factory mag from my 18" works fine in both rifles. The factory mag that came with the 24" needs extra care to ensure it is seated the 24", but works fine in the 18".
I purchased a couple "used" surplus, no-name mags when I only had the 18" rifle. Both work fine in either rifle.
I later bought a couple more from a different supplier. They looked fine upon receipt. Since I already had 3 functioning mags, I didn't get around to testing them for several months. When I did, neither would work. One was missing the tang that locked it in the rifle. The other just wouldn't feed, or would jam on every round. I suspect a follower issue on that one. I have new replacement follower and spring kits from DSA, but just haven't taken the time to tinker with it. I will keep the parts from both, for future use.
I subsequently purchased 4 DSA mags that work in the 18" , but only three work in the 24". One of those that did work needs extra attenion to ensure it is seated in the 24". I guess that rifle is a little pickier.
I noticed some of the new metal mags seriously scratched and gouged brass and bullets, especially in the 24". Sanding or filing the feedlips would probably help, or just using them for a hundred or so rounds, but I ordered a couple of the plastic Thermold mags to try. I initially was leary of the plastic/Kydex, but ended up pleasantly surprised. I liked them so well, I bought 3 more. They all work fine in both rifles. One does require a bit of care to ensure it is seated in the 24', but no damage to brass or bullets from any of the Thermolds.