Well, it depends . . .As I am new, I have not had to deal with this yet. Don't we have a choice, even with insurance, where we get our supplies from?
If you have the financial resources, you can go ANYWHERE you want. Our hosts at CPAP.com are wonderful to work with, and I highly recommend them if you can't find a local DME who will provide what you want at a fair price (Brick and Mortar DME's will almost always OVERCHARGE you if you're paying out of pocket--a few won't, most will).
If you don't have the financial resources to pay privately, or you're just not willing to do that because you already pay for your insurance and DME coverage, whether or not you have a choice of suppliers depends on your insurance. HMO's generally give you no choice. For example, my HMO is Kaiser and it's Crapria or nothing. I can't really afford to pay for my own equipment, so I'm stuck with Crapria. Sometimes I do pay for my own supplies just so I don't have to put up with their crap.
Other insurers may give you a few choices and you get to pick. If you're still on private insurance, you may have some leverage. If your doctor is well known to the DME's and regularly refers patients, you may have some leverage, if you know what your insurance will cover and don't let them play games with you, you may have some leverage.
But the story may be different on Medicare. Medicare has narrowed the choices of DME's considerably by adopting the new competitive bidding process. Usually there are only 2 or 3 DME's in a given market, and because of the competitive bidding process, the DME's who won the bids are going to slice everything to the bone to squeeze out any profit. So the process of getting the equipment you want is changing, and what worked a year ago will no longer work. You can't play one DME against another, because for the most part only the "big and bad" players had the ability to win the competitive bids. They are going to preserve profits by offering only limited formularies for which they make the most profit, even if your individual preferences aren't satisfied.
Technically you ALWAYS have a choice, but in reality it may not be much of one.