If you have been keeping an eye on the Welovefine page at all, they have expanded their horn selection to a Twilight Sparkle colored purple horn, and are offering it for free with the purchase of specific Twilight shirts:
The Sparkly Night
Just A Test
Time for Science
Twilight Sparkle Cutie Mark
And on top of that, it looks like they have an achievement system in place for people that purchase multiple items. They have a pretty simple diagram to explain it, head on over here for that!
Thanks to Robert for the heads up!