I'd certainly call S&W first, but anyone can put most of the parts together (Gun Parts Corp has had all the pieces at various times).
On that note, there's a chance there'll be a surplus-dump of all the 1522P short-barrel parts that S&W can't really sell right now, seeing as the brace law-thing sort of stopped sales cold. S&W'd probably rather turn money rather than hanging on to a bunch of parts they can't use to build new braced pistols.
https://www.gunpartscorp.com/products/691770
https://www.gunpartscorp.com/products/685760
https://www.gunpartscorp.com/products/687690
...and so on. Just search 15-22 M&P and you'll get six pages of parts.
I've got a few extra uppers in various configurations, including one of the suppressed integrated ones S&W sent me. Personally, I like the 1522P length. Shorter is cute, but not enough to hold properly or mount night optics/IR aimers.