I have not been able to find bronze pipe fittings either. I was looking for some fittings for plumbing
on the inboard side of the valve, but still below the waterline. Conex make some that looked suitable but depending on where you looked they were either called bronze or red brass.
I asked Conex tech support what the composition was and if they were suitable for marine
use. Here is what it is in them ...
Our red brass aligns with EN 1982, alloy CC499K which is:
Copper – 83.0 – 86.5%.
Zinc – 4.5 – 6.5%.
Tin - 4.2 – 6.0%.
Lead – 2.4 – 3.0%.
Iron – 0.25% maximum.
However, regardless of the content, the tech said they are not recommended for marine use.
On a different mission looking for a valve, someone suggested a local plumbing supply. (hmm, unlikely I thought, but go have a look). The valves were supposedly bronze and looked OK, but a magnet showed the spindle was not stainless, so not worth the risk.