I agree. RMoM for example offers the LaSalle, 49 Merc and 53 Ford pickup transmissions for the flathead... I can't find anybody else that offers anything similar. That's just one category. There's a guy on Shapeways offering a Muncie with a flathead bell housing for almost $13 and VCG is working on a top loader for the flatheads they got from Chris Bolide, which the flatheads look great and it'll be one more option.
Casey at Forward Resin no longer offers his Artillery Wheels because of lack of interest and he does have a beautiful looking set of Spyder Mags but it seems his focus is going towards the modern larger wheels for the most part.
I recognize this hobby is mostly adults and there are fewer of us focused on more traditional builds... hence why Revell has never offered a more traditional option for thir 32's as far as rearends or suspensions. Still, it seems like there is enough interest in "retro" hot rods to warrant a few aftermarket options.
Mapleleaf Modelworks has a few options, I've been communicating with Josh at VCG Resins and I'm hoping he'll be putting out some more options as well. Oh, and I can't forget Jason and his louvered parts or early Halibrands and the items that Ed Fluck Jr still has...
Maybe someday something like this will be available again...