Find spare parts for a makeshift guitar. This is particularly useful if you're looking for more of a stage prop than a functioning instrument. Start out by calling local guitar shops to see if they have any spare necks or spare bodies; they don't necessarily need to match for your purpose. If you can't find any, you can order these parts directly from a guitar manufacturer.
Assemble the smashable guitar. In most cases, you can superglue the guitar body and neck together in a fairly straightforward manner, but if the body has holes in the space where the neck will be, use screws instead to bolt on the neck. This will make it slightly more difficult to smash, so it could make that part of the show more intense.
Paint your assembled guitar. Purchase a can of whatever color spray paint you want to use. Spread old newspaper all over the floor or workstation to pick up any excess paint. Use the bandanna to cover your noise and mouth, and then paint to your heart's content.
Search the pawnshops for a cheap guitar. If you can't seem to find any spare parts, rest assured; most pawnshops should have cheap guitars that are not only smashable but also playable. They may only set you back about $30 to $50, which is a small price to pay for an intensely memorable performance.