BlackCurtains is right, Clipper's Sail is considered the 'main' HQG1C distributor site! But his co-distributor is KatzPonyShop (which I believe is Wysteria on the MLPArena?), and she has her own site with some of her own exclusives, which you can find here:
https://katzponyshop.ecrater.com/c/2092337/g1-hq-collector-club-ponies Katz occasionally gets exclusives like the 'alt hair' Artistry Girls (each line of their mane is done in one color, instead of alternating stripes), and she was the only seller for the Alt Kitty Love that came out last year, for example. If Clipper sells out of something, it's always worth checking out Katz of AHorseofEveryColor to see if they're out too!
There's also a UK distributor, but I think generally the best prices/fastest releases will come directly from Clipper, since he's the primary supplier! However, you're more likely to get pre-dyed blanks from AHorseOfEveryColor, since they specialize in dying blanks, and you only see dyed ponies come up occasionally from Clipper or Wysteria. (I got a blood orange sparkle dragon from Clipper!)