Nothing I know of at Niagara Falls but if you are willing to drive about 30 minutes, there is a kosher deli in Amherst, just outside of Buffalo. I was there about 18 months ago when we got ticket to a Bills football game and it was a small takeout deli with lots of kosher groceries. Its hidden in one of the big local grocery markets so you have to do a bit of searching to find it.
On the Canadian side there is a Chabad situated at 6417 Main St. near the falls which has a fleishik and a Milchik restaurant. They are open at separate hours from each other and are under the hashgacha of the COR (which is the major hashgacha for the Toronto area). Check their website http://www.JewishNiagaraFalls .com.
On the American side, the nearest location for kosher food is Nissan’s Deli which located inside the Tops Supermarket at the corner of Maple Rd. and Bailey. That store has a selection of packaged kosher foods and some of the breads are Pas Yisroel baked in the store. It is under the hashgacha of BVK (Buffalo Vaad) which is a small, local, but well-regarded hashgacha. Enjoy your trip.