Any junk yard with old 6 volt cars will have horn relays. Not sure what you mean by "BOSCH" type relays, but many surplus stores have an assortment of relays. The "AXMAN" and "ABC ELECTRONICS" in Mpls. have many relays.
Here's an example--just a standard aftermarket NO/NC relay, often offered with a socket with five wires.
Bosch Relay
Joe