I have done many rehabs for 1-4 unit properties, and have always just purchased material from local hardware stores. I am starting rehab for a small apartment building and will be renovating all the units at the same time. I have thought there must be a way to order materials in bulk to get a better deal now that I am doing larger volume, but haven't found anything obvious. 

I have found a Chinese manufacturer that will ship me a container full of counters, toilets, tubs, etc. and the discount is pretty significant, but between the low price and some language barrier, this gives me some concern about quality, correct measurements, etc. 

Those of you that do rehabs on several dozen units at a time, where do you buy your material?