I can make a 'weapon repair kit' with 50 repair and very common items. It is not especially finite. You can easily acquire the components while doing your normal play. Duct tape, scrap electronics, wonderglue, scrap metal and wrenches are everywhere. You just need to know what to grab.
Unfortunately, some WRK components- especially the Duct Tape- are not common enough. From my observation, you can find enough components for about 120 to 150 WRKs over the course of the game. You need to burn 3-4 of them to repair one weapon from near-broken to perfect. If you do it a lot, you'll burn through your WRKs quickly- and don't forget that for some weapons (missile launchers, miniguns, etc.) WRK is the only cost-effective way of repair.
Fiends etc are certainly a resource but they need you so spend ammo, time and supplies. One must count all costs.
The expense of hunting Fiends in terms of ammo and supplies is negligible. Ratslayer with 5.56 HP ammo and a Lever-Action Shotgun for backup does the job just fine. All the supplies you need you find on the Fiends themselves. And let's face it, time spent hunting Fiends is time well spent
And you get tons of good karma as a bonus.