Andy,
If you find someone willing to discount for sapwood - please let me know! Like Philly says, it is getting harder all the time to find boards without much sapwood as the trees being felled are smaller. Your best hope is to find a yard that will let you select it yourself - even then they will quite likely add a premium for very good boards!
Fortunately, if you really can't live with sapwood colouration, it is easy to stain to a matching colour with Van Dykes crystals. However, I think it is better to accept sapwood as something to be incorporated into the design and celebrated rather than suppressed, especially as it is what gives life to the tree.