Guidestar is well-known, and has the last 3 years available free for anyone with free membership
FoundationCenter has many 990s as well and has more than 3 years available
990s provide lots of info about who the board members are, who are highly compensated employees and who an organization does business with,
Guidestar is a great resource! You can also request 990's that go further back than what Guidestar posts.
Do you need to pay for older returns?
No, you can ask Guidestar to get them or write directly to the IRS.
This is great information. For much of the research into the adoption industry the IRS 990 forms are an excellent source of information. Often times I had wanted to have older forms, not knowing how to get them.