It used to be much easier to find group/club buys of grain sacks because some wholesalers would sell directly to the groups. Now it doesn't seem like any of the wholesalers are willing to arrange the occasional pallet of grain at the expense of making their LHBS buyers upset about the competition. Now it's pretty much impossible to find a place that sells in bulk that is substantially cheaper than buying by the pound, especially when you factor in shipping. It isn't cheaper for LHBS to buy a sack of grain if they sell it by the bag or by the pound so they might as well sell it by the pound.
If a local brewery is willing to let you buy from them then that is probably the cheapest option.