So promag sells a 20 round drum mag, I personally am not a huge fan of promag, there is a YouTube review of the promag by a guy named "Ron Quixote" and he was saying seating and unseating the mag can be a little difficult.
Google search "ProMag Saiga 12 Gauge Drum Magazine, 20 Rounds" and there are a few sites selling them, there is also a 25 rounder google "SGM Tactical saiga 12 gauge 25 round drum"
Ithe SGM goes for about $99 where the Pmag is $75. I'd personally go with the sgm.
Good.luck and stay safe.

budsgunshop.com/search.php/type/Magazines/q/saiga%2012/sort/relevance ------All of these will fit that model, but as Paul noted there can sometimes be issues with Promag. I personally haven't heard as many recently as I used to, but I'm not sure if that is people steering away from them, or they have corrected the issues.

Scott, budsgunshop.com/product_info.php/products_id/723000386/sds+lh12+10rd+magazine ------------ Saiga 12 Mags/Drums should work as well : -- drummagazines.com/MAG-SDS-SAIGA-12-STYLE-12GA-10RD-CLR_p_670.html