If you mean the Cellulose Humbrol thinners it's a surprisingly complex mix. I had a chem eng mate who worked where they made it. You'd think it would be little polar molecules, acetone etc, but it's not.
If you buy 5 litres it's not an unreasonable price, so you could try asking your friendly neighborhood car spray place to fill a small tin for you. It comes in different "speeds" - which they choose to suit the paint type and thickness they need.