A little bit of money (spent at your LBS) might save you a lot of time (spent on bikeforums, measuring tools, internet suppliers, and waiting for FedEx or UPS to show up). There's way too many "standards" to be able to guess which headset bearing to order. It would be worth it to me to see if your OEM also sells spare parts, or take it to the bike shop and let them play "order and fit."

Of course, your financial situation and/or valuation of your own time may vary.