I remember looking for BioHunter cels a while
back, with some success...

The company that released it in the USA (Urban
Vision) used to have a lot of cels from the
movie that they were willing to sell to fans
for $30-$80.  Some of them were REALLY nice
(I really liked one of Kamada, but wasn't willing
to "commit" to the series in my collection) and
some of them were just B-quality... I think they
also had cels from Twilight of the Dark Master
and a few others for around the same price range
(pretty reasonable).

However, some anime online shops (Taro) will
get these cels and sell them for a hefty price
(several hundred)... so I guess it all depends
on what you want, how long you want to look for
it and how much $$ it's worth to you.

If you search for Urban Vision's page, you
can probably email them and ask if they still
have some cels (I saw the cels about a year or
so ago)...

Hope this helps.  =)