I was with my girlfriend, going to the grand Canyon National Park and she saw a sign about this, asked about it and this is what we were told.
Any U.S. Citizen or permanent resident with a disability gets a FREE LIFETIME PASS to all national parks.
This is usually $80 per 12months.
You also get half off camping and you get to drive down shuttle-only roads with some accessible parking being available.
This is amazing. You may be able to find this online, but if you happen to be near a national park, you could go there and inquire.