Lowboys are getting harder to find. As an aside, if your water was hot, why would you need a heater? Now, maybe calling it a cold water heater might be more appropriate, since cold water getting heated is a nice idea, but it's really just a water heater. It's one of those English teacher things that just throws up a red flag and almost hurts...
The codes now require (nearly?) all residential water heaters to protect against vapor combustion - i.e., it will prevent flamable vapors that might be around the WH from igniting, or if it does, restricts it so the flame doesn't get outside of the WH. This is to prevent you from blowing up the house if you happen to store some flamable liquids near the WH. This means that for the same volume WH tank, it will be taller since the new stuff has to enclose the burner assembly. Electric low-boys are likely more readily available, but your cost to heat will often be 3-4x what gas costs. So, your choices are likely to be really restricted, if they exist at all. Big diameter, low height is less efficient when heating the water, too, and recovery times would likely be slow. On a gas WH, the length of the heat exchanger (and flue in the thing) determines how efficient it can be...make that short, and it may not be able to meet the minimum federal efficiency standards or it becomes MUCH more complicated to manufacture and design.