Blogs:
Wheelchair Wandering (WA & beyond, esp: 5/17, 5/18, 8/18, 11/20, 4/21)
Rolling Washington (Western WA)
Tales of Trails (national)
Accessible Nature (national)
Online organizations:
Outdoors for All (WA wheelchair and bike rentals)
Disabled Hikers (WA guides and programs)
Ian's Ride (West Coast, annual multi-day trail ride to promote accessible trails)
Health and Outdoors Initiative (OR programs)
Books:
Rugged Access for All by Chris Kain (national)
The Disabled Hiker’s Guide to W WA and OR by Syren Nagakyrie (upcoming) (Pacific Northwest)
Online videos:
Articles:
Lists:
Trail Databases:
TrailLink (national (former railroad) trails, filter for accessible trails)
AllTrails (national trails, filter for accessible trails)
WTA (WA trails, stay tuned for improvements to accessibility)
Also, look for webpages from state and national parks, webpages from forest lands, bike trails, and closed roads.