How to find private companies in a Niche field

I'm graduating from University within the next year, so naturally i've been applying left and right. However, i'm finding it quite challenging to even find the right places to apply, considering most of the companies in the field i'm looking for work in (oil&gas trading) don't make details regarding their existence very public..

All that being said, I would immensely appreciate being pointed in the right direction.

Look forward to any and all responses.