There are a number of sources out there for bumping the capacity of the Glock 21 mags. You can go with replacement floorplates and springs which increase the capacity by either 2 or 4 rounds.
As already mentioned, Kriss makes an extension for the Glock mags that increases capacity to 25 rounds. I assume you can buy just the extensions, and I see that at least one retailer is selling the extensions already assembled onto factory Glock mags.