How many brands use own drivers?

KEF UniQ

Vandersteen make the cones and Scanspeak integrates those w custom designed and Vandersteen patented aerodynamic basket ( think about how the rear wave off a midrange runs into the magnet and IS reflected right back thru the cone....out of phase and now lagging the initial signal....unless ya fix that )... magnets, voice coils, faraday rings of various configurations, high temp formers.

on the lower end a combination of custom specifications from Seas and Scanspeak.

I would refute cost savings asserted by doing everything in house...squeeze cast baskets or even milling machine time ( cost of capital ) is not cheap....share those brutal costs w others and innovate where you can make a unique difference...

Reference 3A

Some Spendors ( D-line).

As far as I know, ALL electrostatic (many) and magnetic-planar (Magnepan, Eminent Technology) drivers are manufactured in-house. If not, each company's are made for them exclusively, unlike most speakers employing dynamic ("cone") drivers, Vandersteen being, as others have stated, a notable exception.