Hi Vanessa!
Two words.....WELDING OXYGEN!!! You can ditch the hassle of getting a doc to write the script (correctly) and then dealing with the A holes at the oxygen suppliers by going the weld ox route as many of us here have been doing for years!!
Medical O2 comes out of the same faucet as weld ox....the difference is that med tanks are vacuumed prior to being refilled while on welding cylinders they just crack the valve and give it a sniff test in the event that it's been contaminated with acetylene.
Some welding gas suppliers will lease you a large tank for a small monthly fee (I paid $6/mo) with a deposit ranging from $100 to $150 and refills from $18 to $23.
Some suppliers, like my current one, won't lease.....in my case, I had to buy a cylinder for $300 and refills are $20.....drawback is that the tank is large, very heavy, and hard to schlep around, and, you can only exchange it for a full tank at the supplier where you purchased it....oh, and you NEVER want to let them know you're getting it with the intention of breathing it!!!
The other associated up front costs are for a welding oxygen regulator fro Harbor freight for around $70 and a good non rebreather mask....I highly recommend the ClusterO2kit from our Sister site clusterheadaches.com for around $25.
Dallas Denny