Try EBay.

Titleist does not routinely provide them anymore with new clubs unless you request them at the time of purchase since many of the new purchases are for replacement upgrades and people already have several of the wrenches. They are easily found for sale on the internet. If you have a good relationship with a store or pro shop you routinely use, they may be able to get you one, especially if they have extras laying around. In a bind you can use a regular hex wrench as long as you are careful not to over-tighten.

Check the local golf shop. They sometimes have extras from trade ins, and they might just give you one. Otherwise you can get a torx bit and do it if you can't find the wrench. Just don't over torque it.

Any newer torque wrench that attach the shaft to the head will do. The ones from Titleist, Cobra, cally and TM all torque down to 40 lbs. Why, because the same person holds that patent. There are older wrenches from TM and cally that only torque down to 30 lbs. But Titleist didn't come on board the adjustable head/shaft combo until everyone went to 40 lbs. of torque to secure the shaft onto the head.

Check around golf shops and even any of your golf buddies. The screw is standard so any manufacturers wrench will fit the screw.

Send me your address, I have about 8 of them.
I can send usps.