I've never seen a Petrol stam on ebay, not for the last 8 months anyway... And I'm sure if there was one, bidding would break retail price. Definitely go with the cobalt blue stam, that's your best bet. Good luck!
As for other countries, definitely worth a try. The last retail purchased stam that I know of was bought at the Taiwan boutique, I think? It was early summer last year.