You could and should purchase your domain name as soon as possible. Anyone can own a domain name, you dont have to be a corporation. You simply register the domain name using a registration service such as Godaddy.com or Register.com.
Pay for the domain name using your own credit card, since you dont yet have a corporate bank account.
When you get to the step where you put your contacts, you can put yourself or put the name and address of your corporation if you have already set that up. If not, you can always update your domain contacts (WHOIS RECORDS as they are called), at any time.
Last, next year when you renew your domain name, you will have your corporation running with its own corporate bank account. At that point you can pay for the domain name with your corporate bank account to keep expenses organized.
BEST OF LUCK ON YOUR NEW COMPANY!!!