Hello,
I think the term you are looking for is "non-production"
cels : cels that aren't used for the anime but for something
else. Most of the time, they are done by the same studio
that produce the anime. (so they are perfectly legit).

If a cel is especialy made for a produce (LD/CD cover,
poster, artbook, magazine, rami card...) and if it's
a legit one (they had the right to do it), it's called
an hanken. Such cels are very expensive because there
is only one used for a given produce. They are also of
much higher quality than regular production cels.

Cheers,
Yann Stettler