Have your local Pro or Golf Shop contact Titleist or the local Titleist Rep, they can order shafts directly from Titleist, also they may still have some around or the D+ limited shaft.

Searching for Diamana Whiteboard brings up a few options. While your improvements are notable, that shaft most likely broke from fatigue. The X version would not be less likely to break and that same shaft is likely going to be used. The choice between S and X is more of how they impact dispersion than not breaking. You could have a golf shop go through current shaft options to fit to your head. Better to get the best option rather than spending money on an empiric selection that may be hit or miss.

A new shaft may be half the price of a new driver with that shaft. Might be time to upgrade the whole unit.

All Titleist shafts for drivers will fit from the 910 all the way to the TSi
Honestly the cheapest route would be to buy a used 910D with a stiff or X stiff shaft. you can probably get those for around $75. The shaft itself will cost you around $100.

I know the case, I snapped my X stiff Even Flow and was looking at getting a new shaft. Luckily the guy I bought my 913D3 from gave me the original Stiff damiana as well.

I ended up buying a used TS3 off ebay, and it came with the Hzrdous Smoke stiff. Which is a stiff+ so its almost a X, which ended up being better for me probably. My ball speed is only around 155-160 mph. So the pro shop told me I probably would not need a X for now.

LOL you must have been swinging pretty darn fast! let me know your secret. If its from a 915, i think you could find one thats got a titleist tip on it pretty easily and relatively cheap on eBay, Just type in the name of the shaft, should have a few come up

Sometimes they will develop a wear spot on the shaft as it rubs on the bag divider. Weakens them of it wears too deep. Keep an eye on the other. Like fellow TT'er suggest, used eBay or Golfworks.

eBay might be a good shout! So many decent shafts on there.