I am looking at Titleist promotional booklet showing Model and Shaft Specifications and it shows that the Mitsubishi Diamana D+ White 90 HY is a stock hybrid shaft available in a stiff flex.  It is available for the 915H in 18*, 21*, 24* and 27*.   It would appear they have chosen not to carry it.  

When you look at retailers that carry 7-10 OEM brands, and all the options, they tend to carry products that would be the most common for that product.  As a LH, many of the products aren't even stocked even if available.  Going for a fitting opens up all possibilities.  My first recommendation is to get fit, second would be a reputable site that will order any custom setup that suits you.  I don't recommend off the rack or similar on-line options.  My own experiences with retailers that do fittings is they tend to encourage on rack products.  I'm fortunate that I have a great fitting center in town.  There is a pro shop in Oregon that supports an internet site - Dan's that orders whatever you ask. If you only have short drop-down box choices, go elsewhere.