Best source for shiplap and framing

Hi Folks,
I make sheds and sorts to order, but can't get anywhere near to the prices I see B&Q and the like selling their's for. I realise the quality isn't the same, but the customer doesn't always see that.
I'm located in north Scotland, so even if I need to ship it up from the south, and buying bulk, where might be the best source for tanalised shiplap and framing.
Any pointers much appreciated - thanks,
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