GM SI-series Alternators are available with the flat ribbed belts and old-school v-belts.
The best way to get the right alternator is to go to a Pick-N-Pull salvage yard and "harvest" a unit that has the type of pulley that you need. If the alternator turns out to be a "dud", you can use it as a core to buy a new/remanufactured one.
Watch out, though, you may need to "clock" whatever alternator that you get so that the electrical connectors are pointing in the right direction. If you are unfamiliar with "clocking" an alternator, visit your local parts store and ask the "Old Guy" behind the counter.
Do a search for alternator at ack's FAQ (see sig) for more info.