Given the fairly easy availability of good drivers, can we also buy high quality, simple crossovers? Amplifier kits?
Of course.
For example; Solen in Canada, Wilmslow in the UK, Linkwitz in the US and many many many others (sorry for not mentioning al the DIY sources).
Will it be cheaper than buying new. Probably but not always obvious. Will it be as good as the original from a high quality manufacturer - unlikely. Will it be fun - almost certainly.