Usually you have to be around 30 days until due date to receive one but you just call up your insurance to get the list of ones that are covered and you choose.
I called my insurance company and they told me what companies I could buy from and picked it out and gave my I.d number and they shipped it once they verified my insurance
Or call Aeroflow! I put my information in online and got a call back two days later. All they need is your insurance information and your doctor's name and number and they set it all up for you. Some websites I went to didn't accept my insurance but when I talked with someone there, I was able to get the exact one I wanted.