Personally I'd get it from BRS. Great selection of specific fittings you need.
Yeah if you want to spend double or more on it. I like BRS but other then gate valves and unions your better off getting the rest from the hardware store. I priced out the stuff I needed from BRS and it was 3 times the cost of what I spent at Menards not to mention it was easy to take back the fittings I didn't use.
Lowes or HD for me. I can't think of one thing that they wouldn't have for my plumbing needs. What exactly is it you are looking for that is missing? Maybe we can help direct you.
I've had great luck with this place over the years.
http://usplastics.com/