It also depends on how good the merchant is at stocking levels, Grahams at Liskeard 30 odd years ago had a fortnightly re-order program, most normal 15 and 22 mm fittings were stocked by the box 2 box's of each !! if sold out tough you had to wait until re-order time came around even if it was 13 days time, the standard stock answer when asked for something out of the ordinary was, no longer made, full stop, I wanted a length of 108mm (or is it 104mm) copper, was told by Grahams no longer made, I got it from Manor B S in Plymouth, in a week
Plumbcentre, Plymouth had my order for 16 sets of bath mixer taps, baths, basins, w/c's, the whole lot for 12 holiday cottages, I received 4 sets of bath mixers, rest T/F, told them no good get them, they securicorps expressed another 4 sets down from up in Devon some where, I rung Reading branch they had them in stock, but has it was in another area, and they do not do inter area deliveries, the answer was tough wait for them next month, that was NBG for me so I flew my youngest boy up to Heathrow to be met and collect the bath mixers and fly them back
Perhaps now that Graham's are part of the Saint Gobain group they are a bit better, than they were, I doubt it