Where can I find some sane, serious custom factions to use?

[OurKabalEmerges]

The title. It says all.

[Buster's Uncle]

Depends on exactly what you're looking for...

Check this thread: http://alphacentauri2.info/index.php?topic=2502.0  ete has done a lot of reviewing and indexing on the AC Wiki, and compiled an Ultimate Faction Pack.  You should be able to find everything (links) in there.  Good luck.

[ete]

Worth noting that while a fair number of those are somewhat sane/serious, a majority are either not particularly fitting with lore or unreasonably powerful. Still, there's so many factions that even with a majority not fitting what you want there's probably 20+ which do or would with a couple of minor tweaks, and you can browse them on the wiki.

I'd recommend Sigma's pack pretty highly, and Jarl's if you don't mind somewhat high power factions.

[Impaler]

Well your Avatar is the Fung-boy emblem, and even Firaxis considered them a joke Faction.  So I'm questioning your standards of what constitutes a serious Faction.

[Green1]

Sigma and Jarls are some good packs.

Although I really did not find Jarl's to be all that overpowering.

[Flux]

Although it might not be, per se, completely sane...
It still fits in with the lore if you have a bit of imagination.
I made one called the "Reagan Republic". The idea is that one of the colony pods was taken by a clone of Ronald Reagan. Plus it has a nice base graphic.