First, check your alternator/water pump belt. If the belt has failed, that's an easy fix.
Second, you can get alternator at the autoparts store, just tell them your make/model. There are two types, Early (bosch/denso 85A) and later model (Mitsubishi 70A) you'll need to determine which you have. The 91 could have either type.
If you do have to replace the alternator, it can be done by loosening the idle valve on the side of the intake manifold and pushing it aside. The alternator will have enough room to come up through that space, but it might need to be rotated a bit.
I have a few used Mitsubishi 70A alternators for sale, if you're interested. Check the for sale section for one of my threads