You can find them at about any farm store, e.g., Tractor Supply.
They are normally meant for off-road or agricultural vehicles being operated on a roadway temporarily. You might be in violation of some states' statutes if you put them on an RV or TT.
If you are going well below the speed limit on an uphill climb, I think you'd be better off just putting on your 4-way flashers for the duration. JMO.
For example, in Texas:
Slow-moving vehicle is defined as a motor vehicle designed to operate at a maximum speed of 25 miles per hour or less; or a vehicle, implement of husbandry, or machinery, including road construction machinery, that is towed by an animal; or a motor vehicle designed to operate at a maximum speed of 25 miles per hour or less.