I've always thinned my enamels, acrylics and lacquers with Tamiyas' plastic friendly Lacquer thinner.
Its also a great paint remover as it works like regular lacquer thinners but isn't Hot so wont damage the plastic.
I do all my cleanup with the regular cheap lacquer thinners from the local hardware/ auto parts store.
http://www.tamiyausa.com/items/paints-finishes-60/tamiya-maintenance-material-62000/lacquer-thinner-250ml-87077