You can use a car starter motor or a cooling fan motor which go around 1 to 10 HP. Also there are brushless motors on the internet that you can use for bikes.
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treadmil motor possibly
From a golf cart.
Gas or electric?
Assuming electric - surplus shops, online, golf cart repair/small engine repair shops...etc.
Check with the local golf course, see if they have any carts that are just 'toast' visually but still drive.