U will need ur quantities for sure, your trademen will give u a list of what they want. For roofing materials in particular, timber, felt, slates, lead etc the differences can be up to 4k from different providers and don't worry about going outside for suppliers, for a big order they won't mind delivering.
Guys I know came across differences of up to 4k from local providers to providers who were up to 40miles away.
Local quarries are generally good to stone, hardcore, blinding etc, when paid cash.
I would not be so keen on them for plastering sand etc.
But again some will give u a per unit price, depending on the product.
But don't compromise quality either.