You can buy directly from Glock in either one of two ways. If you can get a letterhead order (ultimate use affidavit) from the Chief (if you are state or local) or the head of your office (if you are Federal) then they will ship the gun(s) directly to your agency. Their handout has the language required for the letter, which I think is/was signed under the penalties of perjury. This is easier after the expiration of the ban on the magazines with a capaciity of more then 10, as they required a certification for use on duty.
If you cannot get the letter, you can fill out the LE order form, submit it with your payment, and a copy of your local FFL signed in ink. Glock will ship them the gun and you have to do the 4473 (the yellow form) to receive it. Prior to the expiration of the ban, Glock would only ship 10 rounders with these guns.
Either way, you can buy two at a time. Don't forget to order extra magazines.
Good shooting!