Lipseys, a wholesale distributor, is selling limited version of Gen 3 and Gen 4 Flat Dark Earth Glocks in various calibers. Glock had unofficial policy that other colors can be done for minimum of 20,000 orders and seems like Lipseys is utilizing it.
If your local gun shop has an account with Lipseys, they can order one. According to Lipseys' website, G21 Gen 4 is $687.
Other variations include Glock 17, 19, 22, and 23 in both Gen 3 and Gen 4.