! Welcome newbie !
Eaton Spring Co. has them , top quality , OnLine Catalog too .
Also Diamond Spring co .
The Bendix brakes from a '51 ~ '56 truck will fit , work far better than the Huck (notice how it rhymes with suck ?) brakes it came with and parts are super cheap and available everywhere because GM used the Bendix brakes up to 1967 .
You'll need the entire brake setup including the backing plates , easy to find from Task Force Hot Rodders castoffs .