are you trying to make enlarged negatives( negative-->negative)?

there used to be a film that could do that, I don't believe its made anymore.

now, you'll have to make either
1. an interpositive, to make another enlarged negative
or
2. a nice scan(preferably a drum scan), and then have that digit@lly recorded back onto film. Most labs offering this service use Portra 160NC, due to the lower contrast.

-Dan