It's hard to make any specific suggestions for where to camp along the route, since there are many highways you could take. And we don't know how many travel days you want to take, what you'd enjoy seeing or doing along the way, etc. But there are hundreds if not thousands of places to camp between your place and Yellowstone.
I use freecampsites.net and Ultimate Campgrounds, mainly. I go for all the free and cheap spots. And I hardly ever make reservations along the way (IMO you've done the hard part when you got sites in Yellowstone). If I want to go 400 miles the first day, I'll look on the maps of those sites for CGs along the route; maybe I'll have to change it to 350 miles or 500 miles, but I'll wind up with 2 or 3 possibilities. Then scope out what's available another 400 miles down the road. On travel day, I will stop by the first of my alternates and if there's space I will probably stay. Then I look at the maps that night and decide if I want to make any adjustments for the next day (because things change, I might be tired or feel like taking a different route or whatever).
