If you're looking for collecting purposes, good luck.
If you're looking just because you want the original text, word for word, with the drawings, you need go no further than the e-book "Erdnaseum" edition recently released on Barnes & Noble's website. I think Amazon has it as well. It copies the original exactly, with a list of the known errata at the beginning.
Have you considered trying auction sites? Not eBay, but sites dealing in real valuables, like Sotheby's and the like? The link below is for a first edition up for auction:
http://www.liveauctioneers.com/item/8433190And they have another copy from 2008 published by Dai Vernon. Cloth hardcover in a leather slipcase. But it's actually "Revelation" - Dai's annotated Erdnase. The newest version still on sale would be more comprehensive if you were interested in possibly getting an annotated copy.
You can check in with Martinka & Co. at martinka.com - it's a magic house that also sells rarities at auction.
Hope this is at least pointing you in the right direction.