The best way to find a used back is to locate a dealer near you. Phase One doesn't publish dealer names on their websites but does publish the phone numbers of ASM's which is located here..
http://phaseone.com/Global/Dealers/HardwareDealersMap/North America.aspx
Alternatively you could call the office in melville at 631-547-8900 and dial extension 126 for my pal Robert. He is the incoming and outgoing telemarketer, a heck of a tennis player, who also, btw, will soon be marekting an herbal hair restorative! Robert can point you in the direction of a dealer. Absolutely drop my name! It won't get you a further discount by maybe a small laugh!
Buying a back on ebay is not a bad idea but it's an unknown proposition. We had one case where UPS lost a customer's back, made no attempt to find the owner or contact us for help and sold it to their liquidator. The liquidator put it on ebay, but we found out about it and got it back.. Honestly it's better to buy from a dealer because of the warranty on the back and the support they will offer.
The "agressive upgrade" policy I refer to is only available to people who bought from Phase One dealers. There is some fine print, I don't know all of it and things change. Always best to contact a dealer, Robert, or one of Phase One's reps.
Last I heard, Phase One was selling pre-owned P 20's (16MP square sensor) for around $8K. I think a new P 21 <http://phaseone.com/Content/p1digitalbacks/P%2021.aspx> is not that much more.