The sponsor of 'this' web site!
It's possible that your best bet will be using more than one supply source.
It's generally considered that the parts you get at a local big-box retailer is going to be 'sub-par'. A perfect example would be irrigation valves. Places like Lowe's will carry Rain-bird valves. But they are a 'consumer grade' without any model number. Places like EWING (if you happen to have one locally) and the sponsor of this web site sell what is considered 'professional grade'. In the case of the Rain-bird valves, the DV series is the 'professional' version of what you can get at the big-box stores. (Interestingly enough, based on pricing on the web, the DV valves at this sponsor's web site are cheaper than what Lowe's currently shows for the 'consumer' version). So for valves I'd suggest our sponsor or a local EWING supply store.
But when it comes to some bulk items (like Sch40 PVC pipe), the stuff is manufactured to NSF standards. So you're basically going to get the same thing ordering from online, EWING, Lowe's, Home Depot, etc. In that case, you might come out cheaper at a local big box retailer (especially in view of shipping charges... if you have a way of getting 10' pipe home from the retail store that is).