For bulk hops I use Yakima Valley Hops, Hops Direct and Freshops primarily. Farmhouse Brewing Supply and Nikobrew also have a decent selection.
I wouldn't brew a single-hopped IPA, because I really feel like you need some layers of complexity in the hops. If I had to choose one hop as a base for all my IPAs I'd probably go with Amarillo. But there are lots of other options as well. Nelson Sauvin, Mosaic, Citra, Simcoe, Cascade, Apollo, Columbus and Centennial are all at home in any IPA.