If you want Direct Drive then buy a Leaf 1500W kit, from Leafbike.com or Leafmotor.com
If you want a geared motor, then buy from EM3EV.com the MAC kit.
If you want a mid drive, then buy a Bafang, lunacycle.com
Other reputable online stores are
I think is reputable, good to double check with a simple search
I am sorry that is all that comes to mind off hand, my brains a bit scattered with the smoky air and the full moon.
Please tell us your weight, what terrain you will be riding, any hills, stop & go traffic, flat, on road, in park, are you commuting or having fun.
What is your budget?