If you mean geth armor, you don't. Geth are synthetic life-forms, and you can't get "geth armor" any more than you can get "krogan skin".
Addition: I think the above answer is non-responsive. Yes, you can't wear geth armor or krogan skin, but I don't think that's what you're asking. What you appear to be asking is for the easiest way to buy armors made by "Geth Armories." In particular, the "Berserker X" heavy armor for krogans is generally regarded as the best armor you can get for Wrex.
While you can buy a Geth Armories license after you've reached level 36 from random vendors, it may take awhile to track that down. Then you have to use the save/reload trick to keep farming armors from vendors who sell armors.
The quickest way is the randomly order items from the MSV Wallance after you've beat Pinnacle Station. Note the Pinnacle Station is DLC and it currently costs $5 to download.