I always buy my Missouri Meerschaum corn cobs from
Aristocob - he is a great seller and I've always been impressed with his service and found his prices to be more than fair. (For S&H he will only charge you the cost of one pipe, no matter how many you order - it's worth buying in bulk, which also gets you increasing discounts)
4noggins is also a very reliable source for pipes and tobaccos of all kinds - he has some MM pipes, great to add into your basket if you are already ordering a few blends.