I have bought 3 C8 thus far, one from local night market, one from Aurabuy, and another is Lightmalls XM-L U3 C8.
And what now, all of their tailcaps are broken! And broken means mechanically malfunction (cannot be pressed or stucked), causing serious flickers (yes I have verified that it is due to the crappy tailcap, not other parts).
So I’m really tired with them now and this raise my question: Where can I buy a reliable C8 tailcap?
mine is not technically a c8, but i have an uniquefire c108. its the same size as a c8. 20mm star and 17mm driver, 42mm lens... i have had it for well over a year with no problems, probably closer to two years
xintd seems to be the top of the line too, but i do not have one to comment on
edit: sorry, missed the last line in bold somehow. not sure if you can buy just a tailcap... but you can probably replace the internals with quality components
I have owned at least eight C8 lights, and none experienced tailcap failure.
The most used I currently have is a Convoy C8.
You may just need to replace the clicky switch in the tailcap with a new one.
Fasttech and DX sell them, not sure which ones are good though. I’ve tried a few with mixed results.
Wow, 3/3 bad switch - 100% failure rate? Talk about bad luck. That, or it’s just your thumb is too strong and you crush them uncontrollably… haha…
I haven’t had any bad C8 switches and I think I have had 5 or 6 of them, clones and all, and that’s not even counting the mini C8s (they’re all in good condition too).
Anyhow, I bought my replacement clickies for other lights from DX, they were cheap and of good quality.
I don’t know man, many of my lights have switch issue, not sure whether it is my luck or what. I did not use them harshly and my fingers do not have bone-crushing force but eventually all the tailcap or switch fail. 
Had no switch failing in use, so far I had only a couple problematic switches from the start.
I think that Solarforce switch could be pretty good. 
But if you need all tail cap, than Manafont version can fit…