I have a 72 Cougar XR 7 351 C and need to replace the alternator. What is the best place to get one and any tips on the process? Thanks for your help. Chris
If it’s the original alternator, and/or I had a competent re-builder in my area, I’d have it rebuilt. Otherwise, you can buy a rebuilt or new one @ your favorite auto parts store.
Removal/installation is fairly straightforward. The factory shop manual is your best guide if you plan to do your own wrenching. You’ll never regret buying them.
There are about as many answers to this as there are people posting here 
A couple of points to consider:
The simplest option is to replace with a cheap rebuilt alternator of the original Ford 1G style from Autozone. This uses the existing wiring and external voltage regulator without change. You can move up to very well built 1G alternators such as this one from Summit that retains the 1G wiring configuration but puts out noticeably more current.
Next you can consider the 1-wire alternators that are usually a GM-style back mated to a Ford style front. These are internally regulated and do away with the stock wiring harness and voltage regulator. These have many pros and cons so do your research to see if they’re right for you.
Finally, you can go with a later model Ford 6G alternator which will require different mounts and wiring but will power significantly more accessories.
Disconnect the battery FIRST!
Another option is to rebuild it yourself. They aren’t overly complex, and don’t require specialized tools or bearing presses.
Hardest part it getting the pulley off the front, and you need a torx socket to hold the shaft. You also need to solder the replacement rectifier assembly to the windings. Use plenty of liquid wrench on the all the bolts and let sit overnight before taking it apart. I had my case grit blasted, and bought NOS rectifier, pulley, and brush kit from eBay. Bearings were available from Timken on Amazon. It is now fully rebuilt, looks like new, and I still have my original Ford stamped case. Of course it cost a lot more time and money than just buying a rebuilt.
If you purchase a rebuilt alternator locally, have it tested before you leave the store with it.
Are you replacing because of bearing noise or low output?
If you have the original or any of the teardrop style rear case alternators, that is a valuable core. If you don’t rebuild it you can sell it for much more than the core charge.
As a previous poster said, the toughest part of the rebuild is getting the pulley off. Try breaking the nut loose before you release the belt tension. The closest thing to a special tool is the wire to keep the brushes compressed against their springs in the holder while you put the rotor back into the stator.
Sometimes all you need to do to remove the pulley nut is to use an impact wrench while holding the fan on the alternator with a rag and heavy glove. (Off the car)