There's a few good companies that do after market high powered alternators.
The one you mentioned, 4alterstart.com and excessive amperage. All seem to get decent feedback from users. Mine is a 275 4alterstart product. The only out of about 6 companies I researched to offer a 2 year warranty. Excess amperage is 90 days. I think ohio, was only one year.
If you trying to sell it, try the classifieds section.