I'd suggest staying away from ProMag as well. There have been a few aftermarket magazines available for these pistols over the years, and absolutely none of them have a good reputation for working. I remember when ProMag first came out with a magazine for the P99, that the feed lips were too high, and quite a few P99 owners had ejector breakages because of this. The ones that came later fixed that issue, but were nowhere near as reliable as the factory mags.
The only mags that have a good reputation for working in these pistols, are the factory mags, made by Mec-Gar in Italy. All of the 99-series pistols come with these from the factory. In a .40 99-series pistol, you have the option of getting a 10rd, 11rd, 12rd, or a 14rd mag. I'd stick with factory mags if you want a reliable pistol.
I believe this is the part number for the promag 20 round magazines.
There is a buy now listing for the 20 round 40 cal magazines on gunbroker. I'm not affiliated with them, but they appear to be a top100 seller so they are probably a safe buy. https://www.gunbroker.com/item/828030609
Can't speak for quality/reliability on these .40 cal ones. I just picked up a few of the factory 9mm twenty rounders so I'm excited to give them a whirl.