Garolite is a tradename for a phenolic material. Phenolic comes in a variety of grades based on the amount of fiber that is included in the plastic. From what I have discovered, pehnolic grade determines cost. high grade phenolic used for circuit boards is pretty expensive, about $250 for an 8x10 sheet of .032 or metric equivalent.
If you contact a plastics retailer and mention "garolite" they would provide the same product or something the same by another mfg. Most plastics retailers will cut pieces to size for a small fee. cost per foot would be less from a local retailer then McMaster-Carr or MSC, but then again you might have to buy a whole sheet.
Here is the McMaster-Carr page:http://www.mcmaster.com/