They are not a common item to order, Reps do play favorites on who gets certain models, unless sales are REALLY slow.
I can't say that one dealer over another would always have them in stock, but they are out there if you look.
The P-Series guns usually sell for the same price irregardless of caliber, but if you find a custom shop gun or something special, they do command higher prices.
The 1911 series sells for a couple hundred more as they cost dealers that much more to order.
Oh, and the CPO program is slow to process new guns, so when a dept trades in a large number of P229's or whatever, it might take a year to actually see them back on the commercial market. (historically)
All of the CPO's I've seen recently have been very good quality and well worth the asking prices, as the guns have not been refinished and usually have the original barrel so you can see how much wear is evident. (previously the CPO's would be given a spray on coating, and new barrels, but you wouldn't have a clue as to how beat the original gun was.) Finish issues were found and I think SIG has changed how they process the guns due to customer feedback.