Are the hard to find? - No not really. Most major LFS can get them in. Or many of the major online dealers of marine fishes..
As of ideal tank footprint - are you looking for the minimum recommended? Which books - like Aquarium Sharks & Rays by Scott Michael may suggest these sharks can be kept in a 100 gallon tank. Truth is I would suggest Coral Catsharks(Atelomycterus marmoratus) really need a tank that's at least 6' long x 3' wide (about 200-270 gallons) for a single adult. A small pond of at least 5-6' diameter(300-400 gallons) would be ideal.
IMO - A 240 gallon tank (8'L x 2'W x 2'T) is really too small to keep a fully grown Coral Catshark for the rest of it's life. Because the tank isn't wide enough.
You really need that is wider than the shark's maximum length. In the case of a Coral Catshark - the tank should be at least 30" wide. 36" wide would be even better.
I have an understanding (lots of reading) that cat sharks are usually ~24" full grown, sometimes reaching ~27" max.
Is this incorrect? lots of sources point me in that direction, only a couple say that they are up to 30 - 36" and they were not very reliable sources, so I am confused now.
While true catsharks can range for about 18" to more than 5' in a few case.
But the Atelomycterus catsharks tend to be about 18-27" in length. There are presently 4 known species in this genus of catsharks. Only 2 can be presently found in North American Fish shops. These are the Marbled Catshark (A. macleayi), and the Coral Catshark (A. marmoratus). The Marbled Cat grows to only 24" long & has grayish bands with small blackish spots. The Coral Catshark grows to 27" and appears to have whitish spots or stripes.
Personally I suggest a tank that is at least 30" wide for either species.