Old west waypoints and trails
Some members of historic trail associations have starting taking GPS readings on exact trail locations that they have documented, but that are not precisely located on existing maps. I haven't seen that data published anywhere yet. I suggest you contact the historic trail associations via their websites and see if they will share that information with you. The information is there, but I'm not sure they'll release it before there's provisions for protecting the historic sites from potential abuse.
Unfortunately, some of the best preserved sections or historic trail are on private ranches that are posted. Perhaps that good, because lots of ATV and motorcycle use of the historic trails would accelerate erosion on the unmanaged sections of historic trail. A couple of years ago I put lots of effort in locating some precise locations of Santa Fe Trail ruts only to discover that the wagon ruts had become deep gullies that are still actively eroding.
In the meantime check out the City of Rocks National Monument in Southern Idaho. It's a great place to camp and a fork of the historic California Trail passes through the National Monument. It's not far from there to where the Oregon Trail crossed Southern Idaho. Historical markers are pretty abundant along the routes where the trails intersect major highways.
The National Park Service has a historic trails programs and will give you maps of all the major historic trail routes. The Park Service maps are great at getting you close, but generally not adequate if you want to see actual wagon ruts.
Sometimes we get lucky. When I was riding across Nebraska last summer I accidentally stumbled into a place where my GG grandfather camped on his was west in 1847. It wasn't on the map. There are rewards for getting the best available information then just going out and looking.
Good luck finding somebody willing to share GPS waypoints.