Where can I get a .350 tip diameter shaft?
My suggestion is to find the shaft you want (from the huge selection of today’s tip size for woods, 335), then prep it and the install it with a shim15 (335 shaft + 15 shim = 350) – https://www.golfworks.com/the-golfworks-brass-adaptor-shims/p/shim/
This will work just as well as having a 350 tip shaft.
If you want suggestions for senior flex shafts, you might want to start a new thread.
Hope this helps you! Michael