Hex bolts are pretty standard. It's easy to find in appropriate bolts to fit your rack, and there's a good chance those will also fit other things on the bike, so sparing is easy.

Specialty shops (e.g. Fastenal) will probably have all the Torx bolts you can wish for.

I don't know about security. It's easy to find multi-tools with hex wrenches. You can also walk into a good hardware store and walk out with a pile of Torx drivers. I think Sears has a Christmas pack of Torx drivers. How much security does that offer?