There's no single resource that I'm aware of. Most schools will have the most arrangements with sites closest to them geographically, with other sites located farther away. In terms of the types of sites available, most schools have arrangements with VA hospitals as well as some primary care sites. Your best bet for information is to contact the schools you are interested in directly.
You can contact the school and find out who is in charge for assigning externship. Whoever is in charge for assigning externship can help you out. I don't see why they wouldn't give you a general run down on where the sites.
For Illinois School of optometry
You can contact Dr. Caden for info.
But keep in mind that the externship site does change time to time.