Hi Collin!
The most compatible (and beautiful) LEDs are SK9822’s. Next up is APA102. It’s also compatible with most “Neopixels”, which are things like WS2812B. The full list is here in the bullets at the bottom.
Wizard (Ben, the ElectroMage himself) sells compatible LEDs on the Tindie store.
However, your use of the term “end firing bulb” makes me wonder if you’re looking for a strand, or individual pixels (whereas what’s sold on the store are strips). I like to call any of the individual LEDs spaced out along a cluster of wires a “strand”. We have a great thread on the ones with globes installed here.
Unfortunately I’ve never seen strands of the really beautiful ones, only Neopixels.
Any of these styles (strip, strand, globed strand) might be appropriate to shine behind quartz crystals, so maybe if you have a more specific idea, could you clarify by linking to a picture?
Good luck!