Carpenter is a specialty steel maker. Anything you buy that's much more exotic than plain old chicken-wire grade steel is likely to be from Carpenter.
I got my 4140 from a local steelyard. There are several online sources that will sell a foot or two. As an end user, steel looks pretty much like steel; any legitimate seller should be able to provide a "test report" showing the chemical analysis of the melt the bar came from. The test reports are generally furnished at no charge, but you have to ask for them. Not that you could tell the difference, but it increases the likelihood of the warehouse guy cutting a piece off the correct bar.