I use o2 anytime I am above about 8k as that is when my pulse ox reading indicates o2 would be beneficial. It helps prevent symptoms of hypoxemia and the cost can be virtually nil after the initial hardware purchase.
You want o2 on the cheap, do this:
Go on ebay or craigslist and buy:
1-2 small o2 tanks for the plane (CGA870).
1-2 330 cu ft cylinders (CGA540) to use as supply to refill the small plane tanks.
a transfill adapter (CGA540 TO CGA870).
a couple regulators (CGA870) - i like the pulse 5 regulators (around $50 on ebay)
a couple nasal cannulas.
The CGA540 is the one typical on welding oxy bottles.
The CGA870 fits the small oxygen tanks which I carry in the plane.
You can refill the large tank for about $30 at airgas and use it for hundreds of hours. You will need a transfill adapter to transfer oxygen from the large cylinder to the small tanks which are carried in the plane.
I bought two 330 cu ft oxygen cylinders locally which last a very long time - as in years for me.
I carry M6 oxygen tanks with pulse 5 regulators in the plane. The pulse 5 regulators are conserving regulators which means they will only blow oxygen when they sense you inhaling; this will make your oxygen supply last much much longer.
Transfill adapter:
http://www.ebay.com/itm/Oxygen-Transfil ... 1e7b7755db
Much more convenient to fill at home and just about free to do so.
Oh, and no prescription needed to refill your large welding oxygen cylinder.
As for tank location, I have been flying for hundreds of hours with a couple M6 tanks propped up behind the copilot seat in the baggage area (front of baggage area, against right side of fuselage). From the left seat, I can easily reach the top of the cylinders with my right arm to open the valve and adjust the o2 flow rate.
)