Home Depot building supply stores are one outlet for this brand. The Day-Glo orange paint is applied as two coats over two coats of white primer. The coating of paint will not affect the ability of a magnet to pick up the disk. This is mainly for the benefit of the judges so they can see if a vehicle has wandered over the line. Normally that is not a problem for powered vehicles.
Blimps frequently drift out of bounds, however.
If you are using GPS you should not have a problem with coming too close to the boundary. Note that the starting area will also have its dimensions outlined with tape as well. This did not work out very well because it was too easy to break when people walked across it. Plan on orange surveyor's tape at the boundaries. I am not sure what you are describing here.