I did the I&M from Morris to its western end, then on to the Hennepin canal trail last October. My first night was at Illini State Park in Marseilles. Depending on what direction you're going, it may be a good place to stay. I wouldn't say it's the most scenic state park I've ever been in (not bad for Northern Illinois Standards), but it was more than adequate (including having clean restrooms, a shower and a food stand).
There are also a few (3, I think) camp sites immediately off the canal right by Starved Rock. I'm not sure what is involved in reserving them, but if you're carrying food (and possibly water... not sure I'd want to take it from the river) they'd suit you fine.
If you make it over to the Hennepin, there is plenty of camping along that path. Towns for food and water, though, are a lot more sparse. Nice, easy trails to ride, though.