How so I start a new .Mac email address for my new business?

To add an alias to an existing Apple ID: iCloud: Use email aliases in iCloud Mail

Or create another Apple ID, and use that.

For a business, I'd tend to prefer to use hosted mail with a domain that's been registered to the business. This means that you can manage your own email addresses and associate those with a web site, and—because you're the domain registrant—you can relocate your hosting around as required as your business needs changes. It also means your addresses continue, whether your hosting provider changes their domain names or otherwise. Which has happened. Registering a domain and paying for domain hosting is not free, but low-end hosting packages can be inexpensive. There are many organizations that can provide mail and web hosting for you; I've had dealing with MacHighway. And beyond web and mail hosting, there are other approaches available for shared hosting should your business needs increase. Without forcing your customers to use new domains or new email addresses.

Apple no longer issues @mac.com email addresses for new iCloud accounts. When you sign up for iCloud, you will receive a new @icloud.com email address.

You'd need a new AppleID for one to setup a new iCloud email I think.

Wait, maybe an Alias would work...

https://support.apple.com/kb/ph2622?locale=en_US