Surf the net. there are several places that sell at a discount. My quilt shop also recommends the same. I buy the light gray and use it in almost everything. Bottom line(light gray) in the bobbin. It's costly but it goes a long long way, wouldn't use anything else.
I have a 153 and have used Coats & Clark in it for years. Then bought a 640 tried using it and the lint was terrible and that ended that.