Harbor Freight, Tractor Supply, JC Whitney or Sears would have them. I have had a tank from Sears for a lot of years it's been nice to have a lot of times...
Get an HVAC guy to give you an old freon tank, then buy the manifold kit from auto parts store or JC Whitney so you can fill it. It has a hose and fill valve. Had one for years, works great. Gary
Why not invest in a small Nitrogen tank? You got air for many tires with a small tank. Our Nitrogen tank will fill up three 235/85-16 LRE tires to 85 PSI.
The 12v plug into the cigaretter lighter is way too slow airing the tires. Ideally, when you pull off the track after your session you want to check air pressure. And, with the 12v I have it takes way too long to get 3-4 pounds of pressure into each tire.
I have seen people convert an old 20 lbs propane bottle into a handy portable air tank. It would cost you about as much or more to make one then to buy one, but the quality and safety of the propane tank is much much better.