Plus 1 on Mariner Sails, most (if not all) of the folks that work there are very knowledgeable. I think they do some warranty work for some of the kayak manufacturers? They also do a lot of installation work. Because of this, they have a very well-rounded parts selection. If they don't have what you need in store, they can typically get it within a few days.
Most of their product is priced competitively, I will typically check prices on Amazon and then purchase from Mariner if the price is about right.