You can go all out and get the Speedball Art Products Mona Lisa 1-Gallon Odorless Paint Thinner. The lack of door helps work in smaller studios and it keeps well for a long time, if well capped.
You can also use the thinner to dilute your paints and make washes, but it will increase the drying time, and you will find it takes some time to master the making of hues. However, it can lead to some interesting watercolor effects.
If you are working in a well-ventilated area, you can grab some paint thinner from the local hardware store. Keep it in a mason jar and use it as you paint to help keep the brushes clean.
Note: You should be able to clean the brushes with dish soap after you’ve been soaking them in thinner. Just wear thick rubber gloves so as not to get the paint and chemicals on your skin. It will irritate it and get absorbed into your system.