We use Simple Green
at the barn I work at. It is completely non-toxic, biodegradable, and environmentally safe. It's the only thing we can use to clean the horse's water bucked and feet tubs because it won't hurt them if it doesn't get washed out completely. And it smells good too.
You can get a gallon jug of it at your local Home Depot.. But I'm sure they have smaller sizes too.