There is an outfit in AB that sells them
I don't recommend them, hassle to load, unreliable, and easy to break.
2x 25 round BC steel lips is better IMO if you just want to blast away.
They clip together, you reverse them after 25 rounds.
They load really fast and you can use crank or plunge loaders.
For accuracy, you can't beat the factory mag.
Try ruger 10 rd mags epoxied together if you want more capacity.