You should post your year and engine type so someone could better help you. Here are my suggestions though without that info.
1. If nobody knows off the top of their head I would just get it from the dealer. A bolt shouldn't cost more than $4 at their crazy prices. Unless it is some fancy bolt.
2. Pay the entrance fee $2 to a junk yard, and pull it off a donor car and somehow lose it in your tool bag
3. Or you could take a piece of the bolt and go to a local hardware store and try and have someone match it for you for around $2.