From all the research I've done, the Keos are ever so slightly different and over time as the cleat wears, the engagement gets looser.
I doubt many bike shops will ever carry replacement P1 cleats. It's such a niche accessory, given the price of the P1s. I think you're better off ordering the replacement cleats from PowerTap or buy the OEM cleats (Wellgo RC-7) from Ebay for $10. I picked up a pair of RC-7s from Ebay for when my stock cleats wear out.