The reason you're having a problem finding a load center type fuse box is because (I'm pretty sure) they're not made anymore - they're sort of like NOS, surplus only.
Where are you going to grab the power for your fuse box? If it's from the main house panel, then the feeder wire from the main panel to the fuse box has to be protected; the only way to do that is to install a new breaker for this feeder. You can't run an unprotected wire outside the panel, if you do, your fuse box becomes meaningless. If you have to use this breaker, then why not just make it a circuit instead - less devices along the power path. Either way, I would check with the local building department to cover yourself.
Where are you going to grab the power for your fuse box? If it's from the main house panel, then the feeder wire from the main panel to the fuse box has to be protected; the only way to do that is to install a new breaker for this feeder. You can't run an unprotected wire outside the panel, if you do, your fuse box becomes meaningless. If you have to use this breaker, then why not just make it a circuit instead - less devices along the power path. Either way, I would check with the local building department to cover yourself.