That's a good one! The answer is... it depends, and availability is driven mainly by demand. It's expensive for mills to roll out special sizes, so they're not going to provide steel in a certain thickness unless they know they can sell enough of it to make a profit or at least break even. Custom knifemakers are a drop in the bucket of the overall steel market, so sometimes it's just not worthwhile for mills to provide the thicknesses we want.
For instance, AEB-L is a great clean and superbly fine-grained stainless steel, but almost all of it is used for kitchen cutlery and disposable razor blades... so it's pretty much impossible to get in pieces any thicker than about .130". Likewise, W2 and 52100 used to be very hard to get in anything other than thick sections and rounds (which is fine for forgers but basically useless for stock-removal guys like me), but thankfully Aldo has had a bunch of it run in flat sections that are appropriate for either style of knifemaking.
Typically, the most common knife blade steels are available in thicknesses ranging from around 1/8" to 1/4", and are cut into bars ranging from 1" to 2" wide. If one is willing to buy enough of it at one time, larger sheets can sometimes be had; this is great when one gets into having large batches of blanks water-jet cut.
If you're curious about a certain steel's availability in certain sizes, check with
New Jersey Steel Baron,
Alpha Knife Supply and
USA Knifemaker. All three companies are run by knowledgeable guys who understand custom knives, and cater specifically to knifemakers. :thumbup:
If those cats can't get it, it probably doesn't exist domestically.