Sometimes if someone or some highway is having some split spoon sampling done or rock coring done, you can get the guys on a hydraulic drill rig to help you out for a few dollars. I haven't seen too much that they can't go through (except for some freaky undergound stream of sand - snapped rock coring bits off like twigs). An air rotary would work too. But it all depends on access to the areas, how much work, etc etc.
The one drill Co I worked for before had a ditch witch set up with a hydraulic drill on the back end. Could go anywhere and dig through anything.
Approach the guys directly at the job site. That is how we always made a little side money.