I've never heard Sims be able to buy personal hygiene products that will wash a Sim or help them with their hygiene BUT you can buy with aspiration reward points either a potion that will up your Sim hygiene or buy a trait that either will make hygiene go down slower or not at all (depending on the trait you choose). But the most cost-effective way of getting your Sim clean is to throw him or her into the shower. If she or he is in a dorm, the dorm should come with a shower, at least in the default dorms and university housings that comes with the University pack.
