Something that is rated for the amount of pressure you're going to put to it. Unless your going to hop it, you'll probably want something rated at least 130PSI. 5-10gal. Maybe less PSI with a larger tank. I figure you'd want it to full fill at least once without the pump having to come on. So it depends on the pressure you plan to run.
Feon tanks started putting one way valves on them for the purpose of stopping people from using them as portable airtanks because they are thin and the moisture rusts them on the inside.
I used a tank from an old pancake compressor I had, its a tad small but I little space to work with.LOL
Don't see why a cheap harbor freight or yard sale portable airtank wouldn't work. They are a lil chubby usually but I think I paid about $10 for my craftsman at a yard sale or auction. And I believe it's gauge goes ove 100
ebay always has tons of deals going on that you can pick up tanks and such for cheap. I picked up a refurbished like 5-7 gal with 4 additional bungs welded in, powder coated for 40 shipped. A compressor will come the pressure switch etc, and you can buy panels that have gauges and switches under 50. i personally have about 550 tied up in compressors(2), tank, solenoids, 4 bags etc.