Everyone, including Cimmaron Arms and R&D, seems to like Black Hills Ammo. Try that. They all recommend it.
I tried to look up Maryland gun laws. I couldn't find actual statutes, but I found a synopsis of some laws.
This is going to sound a little strange, but there are some pretty strange gun laws out there.
You have to be 21, in Maryland, to purchase a handgun, but you can get a permit to carry one if you're 18.
Makes no sense at all, but there it is.
According to the BATF, C&B revolvers are not firearms. Cartridge conversion cylinders are considered accessories, so no federal laws concerning them, either. Not yet! If they won't sell you a cylinder in Md. just get it on the web. Just make sure you remove it if you are going to transport it to a range, or wherever.
If you have a drop-in cylinder, and do not alter the gun in any way, the gun becomes a BATF firearm, only when you install the cylinder. When you take it out, it returns to non-firearm status. So, you can't buy a gun with the cylinder installed until you are 21. If you have a loading gate installed, the gun is a firearm all of the time, regardless of which cylinder is in it at the moment. However, you can send the gun out to a smith and have the gate installed, and they can return the gun directly to you, as opposed to having to send it to a FFL dealer. Actually, they can do the whole conversion for you, still without involving a FFL dealer. I don't know how long that is going to last, but that's the way it is currently.
Sorry, but I didn't see anything about ammo. However, you can get factory ammo, or reloading components over the Internet. FYI, they charge a $20 per shipping container Hazmat fee to ship powder and, in some cases, primers.
Right now, the anti-gun crazies, in most states, don't seem to know about black powder guns, and apparently, neither do criminals. In a few states, like the Peoples' Republic of New Jersey, BP guns are considered firearms, and you have to have the same plethora of permits for them, too. Don't ever move to NJ!!!
I assume, since we are talking .38 special, instead of .38 Long Colt, that you are considering a Remington. The cylinder pops in and out so easily that it's faster to reload the cylinder than it is to reload using an ejector rod. In 1869, when Remington started making cartridge conversions, this was the most popular choice. People didn't bother with the ejector rod. Clint Eastwood uses this type of Remmy conversion in Pale Rider.
Hope this was helpful.