There are of course some MOS caps you could get, but
they will go by other names. For example, FET-quad
"passive" mixers are nothing but 4 zero-VT FETs that
are sized for ~ 1 ohm on resistance and the LO+, LO-
terminals will be the two pairs of gates; the other
terminals are sources / drains.
You can also get discrete low- to medium-voltage
MOSFETs and get nF-range Cgg.
If your interest is just learning / experimental, CV
curves and so on, these might do as a test article.
But for real applications that need performance at
price, the last thing you want is to make a cap
using high end semiconductor equipment. Unless
you're already headed down that tube in which case
it's a freebie or an incremental mask, depending.