One store I know has some .38S&W in stock right now (well several days ago) is Elwood Epps, banner above. Not listed on the site, so you have to contact them or drop in. Of course, it'll sell out now that I've told everyone.
Probably other stores have it in stock, but don't include it in their on-line inventory, so calling around should stir up some. There's one near me that has some, but that's my secret. The only .38 Smith I've seen for a while is Winchester in a silver box
There's always old cartridges of it at gun shows, as it was a very common revolver round a long time ago, as well as used in a variety of service revolvers. Many of the older guns, like the Enfields chambered in it, were meant to fire a heavy 200 grain bullet. The new stuff uses like 140gr, so the point of impact is quite different from the sights. Finding a bullet casting mold in that very heavy weight and the correct diameter is also very tough, so the only ammo that has the proper projectiles is on the gun show tables.
Not likely to see much new production of funny old calibres right now; they are going full steam on 9mm and .223, and won't be switching the tooling to pump out any of this for a while.