I was under the impression they can drop like ammo and be bought at vendors, so as long as you had the caps you should be able to pay for it.
I hope this is true as I'm building my entire character around being a brotherhood paladin and the power armor is vital.
By mid to late game they are commonly sold and affordable
Theres also a bunch of mobs fights, where you can loot 6 cores at once. I'm level 16, basically never fight or explore new areas without power armor and have 6 cores in my backpack.
I'm at level 36 and have around 60 odd cores or so without going around actively hunting for them. There's more than enough to fuel both my power armour and gatling laser all across the wastes.
I think its a glitch with the ones you can put a core into and then activate for quests, kind of like the current Infinite Caps exploit, (it does not work on non merchants that I have found that will try to barter with you, but anyone who is a merchant CABOOM they owe me millions of caps for what they are giving me for free(their entire inventory), hopefully that gets patched.