Let's see, you want an obsolete cartridge cheaply and conveniently, right?
The first reason nobody makes original type .38-44 ammo is that, as you say, it was apparently more powerful than current .38 +P, so it would have to be labeled +P+ for which there is no real standard to go by. That would open the maker up to liability for wearing out guns. A few small operators are willing to do that but not the major manufacturers, not without a government purchase order and waiver.
The second reason is, that no matter how fast you crank up a 158 grain lead roundnose, it is not going to be a whole lot more effective. Trends and technology have gone towards light hollowpoints.
Mostly. Buffalo Arms sells a 158 grain lead hollowpoint at 1000 fps at a +P rating. Whatever they do to make that happen, you will be paying a dollar a pop for.
http://www.buffalobore.com/ammunition/default.htm#38spl
Probably just as well. According to Colt at the time, the Official Police was rated for .38-44 and other similar high velocity Specials. But then you could get parts and service. A high end factory load is not going to blow up your OP but steady use will put some additional wear on it. Who will you get to overhaul it if it loosens up? I sure don't know.
I recommend getting some of the BA chili peppers (as Col Charles Askins used to say) for when it really counts. Shoot some for familiarization and zero and save the rest for when your neck is on the line. Practice with something ordinary.
There is always handloading. A +P handbook load will exceed 900 fps with a 158 grain lead bullet in a real revolver, not a test barrel. Lead. Even .38 +P+ pressures are not going to get the velocity out of a jacketed bullet that they will with greasy lead. This will call for some extra cleaning but that is the price you will pay for top performance from what started out as a black powder cartridge.