How to find siblings that have been adopted.?

I am the oldest of 11 children from my mother. Six of those children were given up for adoption at various times through out my life. The first one in 1973 and the last one in 1991 and all in different locations throughout Illinois. There is one set a twins in that group. One of my sisters and I are now interested in looking for everyone because they are all of age now. My mother has very little information on some and has hospital and birth dates for others. Does anyone know the best way we could go about finding them? We are willing to go onto a website that requires payment, but we don't have alot of money and want to make sure it is reputable. Please let me know if you know of anything we can do to help find them.