I buy everything from these guys except the glass which I buy local. Good people. Good prices. Excellent products. Plus straight up info hard to find all in one place. Seems there is a lot of variety in what people prefer for materials. I only use epoxy, it's better, but more expensive than polyester. I think it a better value in the end. I use finishing cloth almost exclusively for new parts because it is very flexible and very thin. Bike parts are very small, and many layers makes very light very strong parts (the thin cloth is easier to work with, I think especially for novices. It takes more layers and therefore more time and patience for any given end thickness, but you more easily get a better glass to resin ratio, and better strength, so it is easy to end up with a lighter thinner part.). I don't keep a lot of various weights and weaves on hand. I do generally keep some chopped strand matt on hand though. Those 3 items, (standard "laminating" epoxy, chopped strand and finishing cloth) will enable you to make nearly anything you will ever want for a motorcycle.
http://www.epoxyproducts.com/