Your best bet is to contact the main merchants like Timbmet, Brooks Bros, International timber, Lathams etc and negotiate the best price. If you buy whole packs of timber and get it delivered direct from the dock to your factory you will get the best price.
Timber prices fluctuate a lot as they are dependant on exchange rates so you will need to monitor the prices often.
A volume rate for timber will not tell you the overall material cost since waste percentages can vary widely depending on grading, size spec and timber species.