Most recent one I found was dropped by a zombie, actually. Not sure which one, since it was daytime and I killed some many in that one spot, but nothing out of the ordinary (regular, tough, cop, swimmer, child, shady, spitter, boomer, maybe a shocker? - no brutes/hulks).
I’ve found lamps/nightlights in houses, dropped by zombies, and in labs. I think in a mansion once, too. That’s all I can think of offhand.
I end up making myself one about as often as I find them, maybe more often, actually.