[Where can I find Steel and Ground types?]

They only seem to be available in the underground network, but never encountered one, I'm struggling with the poison gym leader, any advice or other ways to get steel or ground types?

[Looking for Aron]

You can't catch Aron underneath the staircase. When you are in the Railnet, there'll be an area off to the left past a gate which you may have already found. You can rescue a guy trapped there and go down further, there'll be Aron all over the place. A large metal pole will be down there; you headbutt it and an Aron will fall down.

I'm trying to find steel pokemon to use against the poison gym leader, can you get this aron before you do that?

Also if you have advice on any steel pokemon I can get pre-poison gym leader let me know