I think a PTO generator is a good idea, if you have a tractor any way. Obviously, you are running the tractor to power it, and the tractor may use more fuel than a generator with a motor on it. This however is offset by the lower cost of the PTO run generator.
Wico makes one, and they are available from Northern Tool as well. You may also find one for sale from time to time on eBay.
Be careful that the generator you purchase is the right size for your tractor. PTO HP is usually 10% lower than the horsepower at the crank. Mahindra tractors are usually rated at the crank, so a 26 HP Mahindra is only about 23.5 at the PTO. Check the specs on your tractor to be sure.
I just purchased a 8750 Watt generator on eBay for a great price, it has a two cylinder Honda engine and will be plenty for what we need for the hurricane season here in South Florida. I have all these tractor sitting around, but don't own them, so I have decided to go with a stand along generator.
Good Luck!