The outlets are quite easy to find, when you cross at Lewiston you are on IIRC I-190, and the exit for the outlet mall is about 5 minutes after you cross....Porter Rd. if memory serves forrest. Take a left as you exit the interstate, then your first right onto Military Rd....outlets will be on your right.
My wife and I also found, I THINK called Boulevard Mall. It is in the north part of Buffalo (Amherst, NY to be exact), on Niagara Falls Blvd just south of I-290 (which you need to take to get to the I-90 to Buffalo Airport). Decent little mall, much smaller than the Galleria. Has the usual shops, plus a Macy's. If you only had time for one though, I'd do the Galleria before Boulevard Mall.
Speaking of Macy's, one tip I will give is that if you tell them you are Canadian, they gave us a discount for being out of state visitors. Wasn't much, 10-15%, but it was definitely a nice perk (and that was on top of the sale price we had on some clearance items we were picking up).
Good luck in your shopping!!!
Tim