I have a set of Picato double ball stainless flats on my B2A. This set has been on the bass for well over over twenty years and still sound great but it looks like Picato have ceased production. I believe Picato also made Status Hotwires and looking at the Status website it seems double ball flats are all sold out. I don’t know whether Status are any closer to finding a new supplier. The only double ball flats listed on strings direct are LaBellas which cost an eye-watering £75!
There used to be a clamp widget that works similar to a terminal block but fashioned from brass that enabled normal strings to be used on the b2a, however I recall hearing it was not recommended for flatwounds as it wasn’t possible to clamp onto them.
Sorry I’ve not been any help - I hope you find something suitable.