I ordered BioShield's Natural Citrus Thinner from:
http://www.bioshieldpaint.com/22.htm
Luckily, there were so many requests from painters in my area, that a local art supply store now carries it. I love this stuff - it is easy on the brain cells...but still use the (lethal) Silicoil as a final brush wash as it is more powerful. (Note: Silicoil is NEVER meant to be mixed with paints).