> anyone know where I can find one
In your basement fuse-box.
Oh wait, you're using that one.
Home Depot etc. Heavy electrical aisle, under/near fuseboxes.
Every US/Can service panel has one built-in. There used to be a plastic GE 8-breaker box so cheap you could buy it just for the bus. But they are also sold separately, because if you have a sub-panel separate from your Main Service Entrance, you are required to keep your Neutral and Ground on separate buses, so a standard panel needs an added bus. They usually have some very strange shape which snaps-in on the same-brand panel but makes it awkward for anything else.
You can also buy terminal screw strips and jumper bus stock or just some naked wire.