Metz makes flashes that can link to the SCA 300 metering, the 45CL and some of it's variants are available used for 1-200 USD
From the manual (at
www.hasselbladhistorical.eu), it looks like you need two connections to the flash, one for the SCA 300 metering and another to the PC socket on the lens for sync. Metz also made the Hasselblad flash, which not accidentally looks quite a lot like a Metz 45 CL
As Film_man said, if you don't need TTL, just keep it simple and you can plug any flash into the PC socket on the lens.
The Metz 45 is a bracket mount "potato masher" style flash.
If you want a shoe mount flash, there are a few options, one is the adaptor that fits onto the accessory rail on the left side of the camera. Or, some of the prisms have a flash shoe on top, and finally, there was a flash shoe that clipped onto the metal lens hoods. I'm not sure of its compatibility with the more recent plastic hoods. There is also a handle/bracket that screws to the tripod socket and provides a flash shoe.