Call Myles Kelly in Chatham (973) 635-9000. If they don't carry them, maybe will have suggestions. If not, try calling other building supply stores. Slate is heavy, hence the large shipping cost. Are you looking for Pennsylvania slate or Vermont slate? If PA, maybe contact some quarries?
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Anyone know of a good source for roof slates? The few online sources I see have huge shipping cost, and the big boxes dont seem to stock them.