Any tips on getting markings white for shows without the expensive products?

So I'm 14 and have to pay for all of the supplies and Equipment I need for my horse by myself but I just bought tall boots and a new saddle so I'm broke :( I have a show in 2 weeks and I need a good way to get the manure stains out of my horses white sock and make the sock REALLY white for my show! i've tried brushing a lot and scrubbing with normal horse shampoo but the stains just wont come out! I have no money left right now to pay for some whitener or stain remover so I need some good inexpesive tips for getting those stains out for my show, any ideas?