The SGL-21 is a converted Saiga 7.62, what magazine it uses will depend on how Arsenal did the conversion. If Arsenal installed a bullet guide along with the rest of their 922r conversion, then your rifle can use any standard AK-47 magazine (these are plentiful and cheap usually selling for $10-15 for a 30-round mag). Good 20-round magazines are Yugoslavian ones made for their AMD-65 though these will be less common and more expensive than 30-round mags. Also, Tapco makes a 20-round AK mag that I've heard good things about (never used one personally) with a MSRP of $14.99.
If, however, Arsenal did not install a bullet guide, then your rifle will require a Saiga-specific one that incorporates the bullet guide into the magazine body rather than the gun itself. Original Saiga magazines are fairly expensive and available only in 10-round capacity. SGM Tactical does make aftermarket Saiga magazines in varying capacities from 5 to 75 rounds. While cheaper than OEM Saiga mags, the SGM mags are still more expensive than regular AK mags (30-round SGM mags typically run anywhere from $25-40 each).
Of course, if your rifle does not have a bullet guide, you could always install one yourself as it's a pretty inexpensive part. Looking at the picture on Arsenal's website, I would guess that your rifle already has a bullet guide installed, but their website doesn't say one way or the other specifically.
