Apple gives you the option at checkout when you order online, and presumably in store as well. I know that Best Buy and the carriers are also able to sell it now, so that should cover most of your bases.
AppleCare can be purchased up to 60 days after the purchase of the device.
I bought it about a month after the iPad. You have to run a fairly simple automated diagnostic to confirm everything is working but it was simple to do after the fact. Got an email confirming immediate coverage as well.