Apparently the replacement for the Marathon Racer is now the Marathon GreenGuard tire. Yes, you are correct they are out of stock at most places except one that I see.
Schwalbe Marathon GreenGuard Tire 210000003167
In stock at the Hostel Shoppe in Wisconsin.
www.hostelshoppe.com. $31 each plus shipping. For an alternative, see the private message I sent you.