to get into ag19 you need to get a gasmask and night vision goggles. Getting gasmasks boils down to fishing or looting for mesh, and scrapping police armor for HQ fabric, and weapon scopes for lenses.
Getting night vision requires you to create homemade explosives or a rescue saw, which will let you blow open the vaulted doors in the airbase, point oscar, and the water station. They will spawn all the needed parts for the night vision goggles.
There are a couple videos on youtube that show where the needed keys to get into ag-19 are, but once you get into ag-19, you will be able to find rolls of lead as drops from zombies and as floor loot around the place.
Save 3 lead for supplier, get 6 more to create a biohazard suit, and go into fort knell with plenty of ammo and a spare filter. B14, provided you remembered to grab the key, will have the dossier on the table, simply grab it to finish that.

So it all boils down to finding the key to AG19, as I thought. I already have everything else. Thanks for quick response!

From 1:10 in the video he shows the way to the AG19 key

https://www.youtube.com/watch?v=Rl7Hs1x0Ghw