The Model 100 uses either a 531 or 532 battery, both still available from Polaroid (if the contacts aren't corroded beyond help); make sure you have the correct type, though; the 531 is 4.5 V, while the 532 is 3 V, but they're identical size and connectors and too much voltage could damage the shutter while too little won't run it. IMO, it's simpler to clip out the (probalby green) connectors and wire in a 2xAAA or 3xAAA battery holder; the camera will work fine on these batteries, and they'll be obtainable for the next 30-40 years at least. The 2xAAA will even fit inside the original battery compartment without modifying any of the hardware (I've done it on my Model 350).
The Model 100, with Scene Selector as well as four ASA speed settings, can be used with every 660 series film Polaroid sells -- I recommend Type 667, 690, and 665 (the last will require setting the camera to ASA 75 and then turning the lighten-darken dial about one to 1 1/2 large divisions toward lighten to give good negatives, or about one one small division for the print). The auto exposure in these cameras works correctly (with all films) with bulb flash, but you'll have to use guide numbers and either filters or trickery to get correct exposure with electronic flash (though with Type 667 film, you hardly need flash). The synch socket on the side of the shutter is X synch, and the L shaped slot above operates a switch that sets the exposure circuit for flash operation.