Arda Wigs and Epic Cosplay are both very reputable US sellers. Their quality is wonderful, and while they're more expensive than most eBay wigs, you definitely get what you pay for. If you're going with longer wigs, I would recommend Epic Cosplay as they don't tangle much. Both have larger-sized wigs, so if you've got, say, a head 23-24 inches in circumference, you're set. Even if your head is smaller, they can still fit well. (I have a 22 inch head and they fit fine with the exception of their specialty wigs.)
A seller I've heard great things about is Moemall. They sell Lucaille wigs from Taobao, which are gorgeous from what I've seen. The only issue would be they have smaller-sized wigs, so if you have a larger head, they'll be uncomfortable. I believe they sell out of China, so it might take awhile for your wig to arrive.
I've also heard good things about Fantasy Sheep on eBay.
Conventions for 2015 (so far...): Momocon, A-Kon, Otakuthon, Sac Anime
Cosplay line-up: Mayoi Hachikuji (Bakemonogatari), Nona (Death Parade), Nagisa Momoe (Madoka Magica), Menma (AnoHana), Tsukihi Araragi (Bakemonogatari)