[EricH]

Is this for an FRC application? If it is, you're limited to 4W at 6VDC, calculated by stall torque*free speed, as of the 2013 rules. (That's not a whole lot, BTW... typically, a window motor would be used in this sort of application.)

Otherwise, there are a couple of questions that might be useful to know. For example, are we dealing with a Lunacy wheel or a Plaction wheel (or similar high/low traction wheels), as this may have implications on the torque needed to turn the wheel? What is the cost range you're looking for?

Now, that said, I think you might be looking in the range of what would be known as a Giant servo, something like what can be found at http://hitecrcd.com/products/servos/giant-servos. Some of the larger sport servos or boat servos may also be helpful, particularly high-torque variants (take a look around the Hitec site linked for some more servos that might be a better fit).

A small-scale test with a small servo won't necessarily produce the same results as a full-scale model, so proceed with caution.

One other thing. Maybe you can't find one servo that does the job... but two can. Don't be afraid to have 3-4 servos working together on the same pull if that's the best way you can think of to get the needed power.