The two most common speeds for induction motors used on woodworking power tools are 1725 and 3450 RMP. A 1725 motor would require a 6" pulley on it to turn the same RPMs as a 3" pulley on a 3450 motor, because it's half the speed to start with. Similarly, divide the diameter in half to get the speed on the 3450.
So, a 6" jointer cutterhead should spin at around 4,500 to 5,000 RPMs:
An 8" jointer spins a little faster at 5,500 RPMs:
You have an 8" jointer based on your previous post, so you should be looking at turning 5,000 to 5,500 RPMs. If you get a 3450 RPM motor use a 2" pulley on it and a 3" to 4" pulley on the machine.
Often, there is restricted space around where the machine pulley fits, so smaller is better if that's the case. If the pulley is on the end of the cutterhead where there is no restriction you're all set.
I would call Grizzly or Delta tech support and ask what combination of pulley sizes they recommend based on the RPM of the motor you end up with. As far as sources go, Grizzly sells new motors and their motors will be Asian made, but are very good.
A great deal on a 3 HP motor on Ebay:
Another source for motors is Surplus Center which I have ordered from in the past. Their prices are sometimes half of other sources:
Leeson and Marathon are good made in USA brands.
Be aware of "compressor duty motors" as they are not meant for woodworking machines:
Shaft diameter comes into play when selecting pulleys, typically a 5/8" shaft is most common, bat a 3 HP motor may have a 7/8" shaft. The mounting base is typically a NEMA 56 base, but other bolt configurations like 182 come on larger motors. Not a big deal if you make your own mounting plate. You do not want a C type mounting flange in this case. You do not want a 3 phase motor either. The techs at Grizzly will recommend the correct motor for your application.
You can use a 1 or 1.5 HP motor on a 6" jointer. You should have a 2 or 3 HP motor on an 8" jointer for best performance on face jointing hardwoods. Some jointers have 3 blades, others have 4 blades, more blades equals a finer finish cut for the same spinning speed. So, more is better, more blades and more HP, as a general rule.