On the Medicare web site, there's a page that directs you to all medical equipment suppliers in your area. But as you've read in your Medicare Summary Notice, a new list of suppliers must be used, for certain types of equipment, if you live in one of ten geographic areas in which a new competitive bidding system is being tried.
The list of suppliers in these ten "competitive bidding" areas is on a different Medicare web site page. On this page, you'll see your geographic area listed. By clicking on your area, you'll get a page that lists the names of the suppliers for specific types of durable medical equipment, and cities where they're located. As of now, Medicare hasn't managed to list their addresses and phone numbers (though they promise to do so soon), so you'll have to look them up in the phone book, or see if they're included in the original list of medical suppliers (see the first web page mentioned above).
Remember, though, that you have to use one of these "competitive bidding" suppliers only for certain types of medical equipment. These are listed under "product categories" on the same Medicare page that lists the "competitive bidding" areas. A different web page gives details about the specific items of equipment and supplies included in these product categories.