Yes, don't buy them in Manhattan. Instead, drive just about two hours due north, to the Woodbury Commons Outlet Stores where there is a North Face Outlet Store, or two hours northeast, to the Clinton Crossing Outlet Stores. But since you're going on Black Friday, bring weapons & food supplies, maybe flares to help you find your parking space, and plan to sit at the Thruway toll booth for a good long time.
Okay ... since I have a feeling you really do want to shop in Manhattan itself, for the experience of it, rather than save money ...
Try to get the NY Times before you go, and look for coupons in the paper for Macy's, Bloomie's, Lord & Taylor, Saks Fifth Ave, or other large dept stores. They do some really decent discounting & you can often get a better deal than at smaller boutiques & specialty stores.
For discount boots & shoes in general (though no guarantee they have Uggs), try DSW (that's Designer Shoe Warehouse). The one that I know the best is on the south end of Union Square. Then there's Harry's Shoes, on the Upper West Side, on Broadway.
In the city, Uggs are not as "in" as they used to be among the fashionistas, though they are ubiquitous on teen girls & they ain't losing $$ any time soon. The place to go for them is David Z. There are at least two outposts: One on Seventh Ave, in the 50s (south end of Central Park, couple blocks down) and there's another one on Fifth Ave., near 34th Street. I believe they are also at Lord & Taylor.
The North Face has retail outlets in NY, and probably more than one, but the one I know is on Wooster Street in Soho.
Good luck! Wear comfortable shoes for all the walking you'll do. Take breaks. Bring friends. Plan strategic pee stops. Tell yourself you're having fun.
You would not get me in the city on that day of the year for anything.