The cheapest option is probably ketchup! Rub it on their white bits wet and leave it there for a few minutes, then rinse it off. It smells disgusting but it does work!! Has the same ingredient as most whitening products just more of it and it can be a fraction of the price.
I believe cowboy magic is the brand, they make a spray that doesn't require a bath, you just spray on the spot and wipe offand the stain lifts right off! so for a show it helps just get the little spots off if you don't have time for 3000 baths lol