You can't buy an adapter to do this. You can however have your Titleist dealer send the 910 shaft back to the custom shop and they will replace the tip with one for a 913. Expect to pay between $35-$50 plus any shipping charges that may apply.

I thought you were able to purchase an aftermarket Adapter to work with the 910H/F to work for the 913H/F

There may be, just not from Titleist.  Apparently, there are/were non-Titleist fittings for the 910D/F/H and 913D for after market shafts.  Unlikely even non-Titleist fittings will have one that will be interchangeable between 910 H or F and the 913 H or F.  The best you'd be able to do is update to the 913 fitting.  All conjecture as Titleist is firm about what it supports.