HEDs? HED created the patent of the toroidal rim that Zipp use, and feature in HED's 60 and 90 rims. In terms of rim shape, there is supposedly little difference although Zipps have the dimples to create a pre-turbulate effect (I'm reading about this in a Mike Burrows book)....this is supposed to lower the drag like a golf ball.
Cheaper alternatives to a Zipp 404 are HED Jet 60s or Stinger 60s - both of which can be purchased for just over £600 for the pair. Stingers are a structural carbon rim (you need carbon brake pads) while the Jets are an alloy rim with a carbon fairing. Steve Hed rates the Jets himself for durability.
As spandex says, do you need them? I've heard don't bother until you can do 24/25 minute 10 mile TTs as they aren't effective.
The Mavic Cosmic Carbones are another option at a similar price, built like Jets.
EDIT: You could buy a wheel, pull the stickers off then buy the Zipp rim stickers on ebay - not quite the same but they will look the same unless up close.