Getting the bolt off will be the challenge. As for replacement parts, you can purchase parts straight from Indian on their website, but just about everything on the Scout is a M8 thread and you can get those screws at a hardware store.
Not sure what screw you're talking about, but:
1. The screw that goes into a flanged spacer and engages the rear of the driver seat is a M8x1.25 socket head cap screw, 20 mm long.
2. The bolts that hold the front of the passenger seat are M8x1.25 hex head bolt, 20 mm long
3. The screw that holds the back of the rear seat is a M8x1.25 button head cap screw, 16 mm long
Those are the proper names for the screws, trust me I'm an engineer
