I know a little about electric vehicles (people in that field always look for salvageable parts) and those vehicles require very large (6-9 in diameter) motors, so here is some advice:
Personally, I have never seen stepper motors in the size range you're suggesting. If they do exist, they probably would be very expensive. Fortunately, DC motors in that size range are relatively commonplace. In my opinion, the best
place to get motors of that size is *drumroll*... treadmills. Yes, workout equipment like treadmills are common, easy/cheap to get and have good motors. You may even be able to use the controll panel componets as the speed controller (or just use it
as a speed controller). Treadmill motors are usually DC motors, so you can build speed controllers for them (you can buy controllers for a good price on eBay). You might, or have to, gear them to get the torque you're asking (I don't know for sure).
Hope this helps.