It's a right pain in the arse to get apart, ideally you need to
photograph the assy as it comes apart (there are several
dished washers and bits that need to go the right way round, or
you'll get brake failure, or it'll all have to come apart again).
Biggest problem in dismantling is a thick plastic washer
that seems to have been put in with a press.
So sure you can get a kit, but is a garage going to spend
the time, and be legally liable for unlimited damages if it
fails and causes an accident? Are you prepared to pay for
the time involved?
It's a safety critical part.
A wheel cylinder is a different matter, you will still have
some braking. Not that it's worth putting new seals in an
old cylinder, fluid takes up moisture, which pits the
cylinder walls so even with new seals it soon leaks again,
then you get fluid on the shoes, so you need new shoes.