GW don’t (at the moment that is) make a Slaanesh Herald model, so you have to be a little creative. The most obvious solution would be to convert a Daemonette, but I didn’t want to do that and instead opted for a minor conversion of Freebooter Miniatures Chaos Sorcerer.
The conversion work is minor, and mostly involved making the summoned creature look more like a Daemonette.
Freebooter stuff isn’t cheap, but they are very nice models.
I’ve a few others in my unpainted pile that I’m planning to use as familiars and assassins for my Dark Elves.