I have seen scuba tanks in cars before but i dont know what it will cost to fill one up. Talk to your local welding supply companies and see what you can work out with them. Also set up an acount with them so you get it cheaper. If your going to go with nitrogen let me know. I have a business account with grainger and i could save you a little on your regulator. I just looked at them and can get one for 120 and I paid 200 for mine.
check this out......
YouTube - maicos truck p2=