You could check NAPA. Or you could rebuild the current alternator yourself if you can get a kit. I would not use AutoZone alternators, though. I have not had real good luck with their brand of electrical parts. $253 exchange for a Motorcraft doesn't sound all that out of line, imo.
I am looking for a higher output alternator for my truck at the time. Not sure if you are looking to upgrade or just want to remain stock. There are a few places on the web that offer some good prices and no core charge on OEM alternators that have better output than our stock alternators. I figure if your going to be buying might as well upgrade while your at it. I'll email you or PM you some links.