Centre hubs, whilst they have a part number and are shown on the parts diagram as a separate item were in fact never realeased as an item. Buying brand new means buying an entire new hub costing around £40 each wheel.
As parts run out Adam has told me that the price will go up, I don't know what his stock levels are like, but if you want new ones - now is the time to get them.
Ebay is your best option for second hand ones. One thing is for sure you will have to buy the full cap dude.