Individually, each MSC does not post every opportunity on public job boards. Some clients do not allow this as an example. I list a few options below. Volition has not updated their MSC overviews for many years, but the info there helped me some.
I would recommend not providing your SSN when initially signing up. Unless the MSC requires it just to see listings, you can at least reserve providing it only if you perform for them.
My posts are solely based on my opinions and for my entertainment, contact a professional if you need real advice.
When you get in debt you become a slave. - Andrew Jackson