Well, this might not help you much but there is a hobby shop here in the Denver area that carries a lot of the Hobby Poxy paints, including the Thinner you are looking for.

Thompson's Hobbies & Crafts 303.238.5821---- they don't take credit cards, just cash or check. If you contact them they may for an extra price ship you some, don't know for sure. You can work out the details with them.
Lee