There are many solvents you can use for cleaning your brushes, but many of them are toxic. Here are some that are not:
SoySolv
Daniel Smith
4150 First Avenue South
Seattle, Washington 98134
Phone: 206 223 9599
Web: http://www.danielsmith.com
Brush Flush
7114 Homestead Road
Fort Wayne, Indiana, U.S.A. 46814
Phone: 877 501 4200
Web: http://www.brushflush.com/