Check eBay. I saw two listings this week for NOS water valves.
Parts catalog I have show P/N is 1231105.
Applications are:
65-68 P/8
69 All (w/ threaded tube)
69-70 All B, G (less threaded tube)
70-71 All A, F
71-72 All G
Perhaps local Pontiac dealer with capability to search all dealer inventories nationally can find one for you.
1969 GTO Restoration