PROTOTYPE 2

There is a fixed spawn of a Brawler locked in a box that gives you an upgrade (almost) every time. It's located in the North Eastern side of the Yellow Zone. After consumption you have to play a mission or another form of reloading the map to get it again.

From the comments:
"447:1147 in eastern Oakhurst, southeast of the military base and west of the weird docks."

I may have found a good place to harvest these last few upgrades at end game in the red zone..

There is a large base on the upper left side of the Red Zone, three larger infected spawn there one of which has a chance to have an ability to absorb. (Bonus, if you are still awaiting Heli or Armor upgrades you may get a two for one!)

To do this, fly to the crossroads at 455, 245. After a few seconds the Brawlers will appear. If no ability available, fly due south (borrow a chopper from the base!) When you reach a smaller rectangular base at 455, 450 keep an eye in the bottom right of ypur screen, as soon as it says Chinatown, 180 and fly back to 455, 245. Rinse and Repeat!

If ability is available, drop down and obtain it. (Be quick before the tanks kill the brawler) once obtained follow the same steps as above towards the smaller rectangular base.

This is an image of the map showing the location with the waypoint marker and the smaller base to fly to to reset the main base for farming abilities. (Apologies for not using a screenshot, TA can't find my recent screenshots)

https://www.xbox.com/play/media/zqpnGdQUWL

**Edit to add**

Have the weapons you wish to upgrade equipped, otherwise you will only get XP brawlers. Once an ability is fully upgraded, swap it out so both your equipped abilities require upgrading!