Call Micro Belmont or HRP... they will build you a 1-piece jackshaft for the Vmax-4 800 but not a 2-piece for the 750 (don't even ask them, LOL!) Expect to shell out around $1000-$1200 for the titanium shaft.
I don't think you'll find anybody who has one "used" willing to sell very cheap...Titanium has climbed in $$$ over the last 4-years.