Here is the state park in Illinois that is known for Mazon Creek fossil collecting - http://www.dnr.state.il.us/lands/landmgt/PARKS/R2/MAZONIA.HTM
Resources about collecting: http://www.esconi.org/esconi_earth_science_club/2009/01/mazon-creek-reference-information.html
ESCONI hosts two collecting field trips each year. You just need to be a member of the club to attend. Here is the page with information to become a member - http://www.esconi.org/esconi_earth_science_club/become-a-member/
How do you find out about ESCONI field trips? Just watch the ESCONI website's Calendar of Events or watch our Facebook site for field trip announcements, or sign up to receive the event email-newsletter, or attend General ESCONI meetings or study group meetings where the field trips are also announced.
Also, Lizzadro Museum of Lapidary Art hosts field trips on most years - http://www.lizzadromuseum.org/calendar.html