Where do I get blank medical cards for commercial drivers?

I would go to your local DMV or where ever you get your drivers license and ask them.

Medical ID Wallet Cards (http://www.medids.com/Med-Cards.html)

Please define what these cards will be used for.

You can also download one right from the FMCSA site. Recent changes have taken place, so be sure to read through the information. Are you an owner-operator leased onto a company or working for a company? Either way, unless you've got your own authority, they should be able to send you the forms as well.