Just locate your local welding gas supplier and ask about a K tank (249 ft3, 2200 psi) and a 4K tank - (500 ft3 , 4500 psi) rental prices. The 4k is being promoted to save tank storage, so might be attractively priced. Start asking for parts for a transfil and they might get spooky - esp with 4500 psi. Dangerous stuff under pressure and easy to be ignorant of hazards. One might use a regulator to limit the filled unit to 2100-2200 psi just to be safe. More $$$. Buying a used tank might be ok, but here a K is $30/yr with $150 deposit.
Assembly cleanliness, lubes, leaks etc would concern a vendor as they might be concerned about liability by just selling parts to a novice. They might give you some training and then sell an assembled unit, though. They are just trying to stay in business and keep people alive. They really want to sell gasses and be helpful.
Surprised an Auburn guy doesn't know this stuff.
Oh - I mentioned that a full K tank filling a JD (21 ft3) flight tank can refill it 5 times from 300 psi before dropping below 1500 psi on both. That is 72% of capacity in terms of moles and hours at chosen altitude. That is 4X the rated tank flight hours.
I have not done numbers for a 4K tank. A little more complicated for the calculations.