odds are if Danger Den is sold out of something then their re-sellers whom they supply will not have stock as well. Have you tried
http://www.petwarehouse.com and
http://www.petsmart.com ? I have ordered all of my pumps from Petwarehouse in the past, however my last purchase the Eheim 1250 was 3 or 4 months ago. I am sure one if not both of these sites will have the pump, Eheims are very popular in the aquarium world.