how much of it do you need? how old is the house? is it MDF or wood?
there's a chance you might find a supplier with the matching stock
or just get something close, do the whole wall, and it will not be very noticeable.
I am not good at this sort of thing, but I know someone who uses a combination plane when restoring his historic home
and more modern people who make up lengths of matching moulding with a router.