There are many taxi companies in Niagara Falls. Some do well with telephone reservations, others have stands at major shopping malls or hotels. If you are not in Niagara Falls, the city, use a taxi company based in the city where you are staying or visiting... they'll usually be quicker to pick you up. If you are travelling in groups under 4, a car is fine, if travelling in a group of 6, you should request a minivan (at a slightly higher cost). Handicapped passengers should also request a van.
Taxis can take you across the Canada-USA border, regardless of the taxi's "citizenship", but yoiu cannot take a US taxi betwen two points in Canada, and vice versa.
Rates For Airport Transportation
If you take a taxi to or from one of the area's major airports, here are typical rates (based on a 4-door sedan --not a minivan--with up to 4 passengers):
Buffalo Taxis (ie from Buffalo-Niagara airport) have the following meter rates: $2.30 to start & $2.10 a mile & $21.00 an hour waiting time, as regulated by the City of Buffalo. Many trips to/from Niagara Falls and to communities in Ontario are flat-rated, so ask before you get into the taxi.
Oh, yes, and if your driver was courteous and helpful, it is the custom to tip him (or her)! Tips on tipping.
Find Local Taxi cab companies.