Here are some useful links to find internships 

1) Career Services WolfWorks - this is for students and alumni for internship and full-time positions. 

2) ACS Careers - Has a link called "Find An Internship" keeps a running tab of chemistry internships, Co-Ops, and REUs all over the country. 

3) REU Sites: Chemistry - This is a list of the active chemistry division funded REU sites (Sites with active NSF awards).