Just thinking out of the box, here. Fabrice is probably right, more and more car manufacturers are moving away from vacuum boosters and installing hydraulically or electrically assisted power brakes, meaning fewer vacuum booster rebuilds available in the future. Hopefully it will be a while longer here in the U.S., but has already occured in France.
So what to do, for those that want a stock appearance? Some hydroboost units are fairly compact with the overall length comparable to a vacuum booster, and uses a stock appearing master cylinder. Here is a picture comparing a Chevy Astro van unit to a vacuum booster.
There is one company that sells aftermarket electrically assisted brakes, ABS Power Brakes, which seems to be fairly easy to install, as the power unit can be hidden under the dash and just have the master cylinder showing under the hood. The problem, they have to use ABS's master cylinder, which is not stock appearing. Hopefully they will come out with ones that are stock appearing, if there is demand for them. I believe most units that come on new cars have the master cylinder Incorporated into the power unit.
So, my out of the box thought? Camouflage a hydroboost in a vacuum brake can. I believe the hydroboost units in newer Mustangs may be a little more compact than the Astro van ones, but they come with the plastic reservoir master cylinder, and I don't know what the bolt spacing is on them. Next time I'm at Pick-a-Part I'll take some measurements.