If you live in a horsey area, you'll probably be able to find a "blanket cleaning service" that you can drop them off to. They'll wash, repair, re-waterproof as needed.
Or, you can sneak them into the laundromat. I do that... it takes a few drive-bys to figure out an optimum time to sneak them in when not that many people are around, though
. I try to fold them so they look like (very dirty) sleeping bags, no straps hanging out. Then afterwards try to keep drips to a minimum as I take them back to the car. Then, before I leave I run the machines while they're empty to get excess dirt/hair out.