Can I add insulation to exterior home walls without redoing all the drywall?
There are companies that will drill 2" holes at two or three places in each stud cavity. You're left to repair the drywall if it's done from the inside. It can also be done from outside, through the wall sheathing, if you're replacing siding. In this case, patches aren't really needed. House wrap covers the holes. 

I don't know of anywhere that rents the equipment for that. The insulation medium is often beaded polystyrene, but it could be fiberglass. It's fairly specialized. You'd have to check around your area.