I usually buy them from a local auto salvage yard (AKA: "junk yard"). The guy at the desk knows exactly what I want, looks at his pile for a recently rebuilt one, and the deal is made for $15 or $20. I have converted more than 10 tractors and trucks with these alternators, and have never yet found one that doesn't work. But if I do, he'll trade even up for another one. No core charge either because he normally sells to the reman people for less than 1/2 price of what I just paid.
I just cut out all the middle men, and probably made slightly less employment in Mexico city.
On our big tractors with lots of lights and electrical load, I usually use the Delco SI 12, sourced from the same place for about $5 more.
You do not need a 61 or 72 amp SI 10 for an 8N. Even if you have a new wiring harness, the lead going to the alternator is at best 10 gauge. If it is carrying 72 amps for more than a minute or so, it creates enough heat to soften or melt its insulation and the insulation of the adjoining wires in the harness. A few months ago I rebuilt the electrical system of a customer's tractor which had been burned and almost burned the tractor. Bottom line is a 39, 45, or 52 amp SI 10 is plenty of electrical power for any N tractor.
Best of luck in your change-over. The improved starting in cold weather, improved lighting, and long term reliability are great rewards for your efforts.
Paul in MN