Simple question I got a ton from the cathedral and raided Mariposa but is there any reliable vendor who has them, the fortress in the boneyard doesnt reliably have them

I've checked the Gun Runners (Boneyard fortress), and Zack's holding 500+ Microfusion cells in my playthrough. Have you bought from him already? Not sure if his inventory respawns.

Do you know about being able to 'unload' weapons in a merchant's inventory? You should get your 'look' arrow out, click and hold your left mouse button until a list of commands popup, one of them looks like a clip being released from a weapon, select it, and the ammo from that weapon will appear in the merchant's inventory. Some people can hold quite a few energy weapons, and you can get a fair amount this way. I'd check the Merchant on the east side of the Hub, he sells a few big guns and energy weapons if I recall. Try again at the Gun Runners and see if there's any ammo you can extract from Zack's inventory, too.

Gahh I never thought to use the unload like that thanks a lot. Im just roaming randomly around mariposa farming Mutants now.

Also be aware the end game areas contains a healthy supplies of mircofusion cells as well.