Why are there no medium size air tanks?

I believe the limited selection, at least in the Air gun industry is supply and demand. The distributors have to buy a significant number of tanks to get a good price themselves. They in turn have to sell these tanks rather quickly as everyone does not want to pay for a brand new tank that has 13-14 years of life (when they can get 15). At some point the distributor has to discount the tank for the loss of time. If there were no shelf life we would see alot more variation in sizes. The firemen tanks appears to be a larger industry and due to safety get rid of any questionable tanks and most likely go through a lot more tanks than our small hobby does. As others have stated, the fireman tanks may have the size you are looking for.
 
I would be nice if Air Venturi or Omega came out with one in that size range. I've thought about getting the Joe B. 18 cu. ft tank. I'm still waiting on the Sun Optics designed air compressor shown at this year's shot show. A member on Tom Gaylord's Pyramyd Air Blog mentioned that Airforce has purchased a batch and will debut them in the fall. It looks to be the same size as the AV booster, but the Sun Optics doesn't require a shop compressor as it is a standalone unit.
 
"JJZatgun"I would be nice if Air Venturi or Omega came out with one in that size range. I've thought about getting the Joe B. 18 cu. ft tank. I'm still waiting on the Sun Optics designed air compressor shown at this year's shot show. A member on Tom Gaylord's Pyramyd Air Blog mentioned that Airforce has purchased a batch and will debut them in the fall. It looks to be the same size as the AV booster, but the Sun Optics doesn't require a shop compressor as it is a standalone unit.
No matter what label is on CF tanks in the US they made by two companies, Luxfer and Structural Composite Industries (SCI). Both of these companies comply with OSHA because they supply tanks for the military and government use and NFPA because they supply tanks for fire and emergency services. Their standards are far above what any airgunner will ever need to worry about.

So if you want a 30 minute (45 cu ft) tank just get a Scott, MSA, Survivair or any of the re-branded Luxfer and SCI tanks. The tanks that Joe B, and Air Hog sell are probably made by either of these companies if their claims that they use American made products are true. Air Venturi and Omega may get their tanks from China since a lot of their products come China. I have 4 large (Scott) 60 minute, one (Survivair) 45 minute and one (Scott) 30 minute tanks. I have no worries that my tanks are made to the highest standards.
 
Just so you know a bit more about these tanks, the US DOT must authorize their use in the USA in order to transport or fill them. They do this via issuing a Special Permit(SP) number to the tank manufacturer. As someone mentioned above Luxfer, SCI and also Carlton are the primary manufacturers of these DOT approved tanks. These manufacturers then sell these tanks to a Distributor (ie: Scott, MAS, Omega, etc) who puts their graphics on the tank and sells them. If you want to know who actually made the tank, just look for the “SP” number on the tank and google it. In the above eBay post selling tanks for $250 each, you will see the label on the tank say (SP-10945). Sometimes you will see an (E) instead of (SP) in front of the number. Go to google and search "SP-10945". The first thing that will pop up in your browser is the SP for "Structure Composites Industries or SCI". This is an easy way to 1) find out who made the tank and 2) assure that the tank is legal to use/fill in the USA. When you read this SP for SCI that this tank can only be used for 15 years from the date of manufacture. This is the same with all Carbon Fiber tanks such as this that are built to the CFFC design requirements set forth to manufacturers by the DOT.

On a side note, the DOT has issued SP-16320 to Digital Wave Corporation. This allows Digital Wave to Life Extend tanks that are 15 years old. This will give these end of life tanks an additional 15 years of use from the date of original manufacture. With this approval, I believe that we will be seeing a lot more different sizes of tanks coming available very soon.