Try Hunter's or American Angling Supply. Also, you can get Belvoirdale hooks directly from them via their website. Urban Angler right there in the New York City area probably has them too.
Partridge is the only one I am aware of making hooks longer than 8XL, it is the Carrie Stevens Streamer Hook that is 10XL; however, it only comes in 2/0, 2, and 4 and they are really huge hooks make of very think and heavy wire. The only flies I've ever seen tied on them that looked ballanced were trolling flies tied on the single hook instead of a tandem hook.