They're not available at all parks. A ranger at Greenbelt NP told me Wolf Trap is the closest one that has them. You can also apply online but it's more involved (you need a doctor's note, whereas you can at least sometimes self-certify in person) and he said you have to pay for shipping.

Following. This just popped up on ny FB feed.

You can Google this. But you need proof of citizenship and proof of disability. The pass owner must be present and up to three additional people who enter in the same vehicle.

Has anyone gotten one of these in person in the DMV? What documentation is required? How long does it take to get one? Does the child with the disability need to be present when going in person to get one or can I just take his documentation?