How I found new companies:
1) I saw them referenced on this forum - either in the linked list or through jobs being advertised.
2) See the above. It's #1 by far.
3) Independent schedulers hired by the company who reached out to me through their own database.
4) MSPA Lists (It's been years since I've used this)
5) Facebook Groups (I think I've added about three MSC's due to Facebook)
The keys to get me to sign up for a new company:
1) A job is advertised with very attractive compensation - such that the effort to register is outweighed by how attractive the shop is. I'm not going to "jump" for $20.
2) I can't find enough work with my current contacts, so I register for a few more companies.
Hard work builds character and homework is good for your soul.