I bough a 160 amp one off of ebay for about $90 about a month ago, wired it in with some 1/0 gauge and it works great. Here's the link:
http://www.ebay.com/itm/FIREBIRD-GRA...7a1e8f&vxp=mtr
Keep in mind that since your's is a 98 (like mine), the alternator MUST be from a 97 or 98. I'm not sure if you can get a bracket or not to be able to install an alternator from a 99-03. Hope this helped.