get an account and a good LBM.
I have three LBMs around me. One never manages to charge the same rate for the same item from one week to the next, the other had a stupid ordering/docket/account process that made ordering stuff a pain, and the one I went with has been brilliant, and I still have my account there despite only spending <£500 a year on bits and pieces now.
I used to get substantial discounts on shopfloor prices, anywhere between 20% and 80% (rainwater goods), I could pick delivery slots on the lorry for convenience, and the account managers knew their onions about various kooky construction systems as and when I wanted to do something different. Their advice was brilliant. Over the entire construction period, I probably saved over £30k, maybe as much as £50k.
For plumbing and electrical materials though, for the most part I got those at local specialists as stock was better. And the sheer amount of door hardware I needed (handles, locks, hinges, window latches etc) meant it was worth a special trip to Birmingham to a specialist to buy the whole lot in one go.
Like an LBS, and LBM is worth their weight in gold.