I've used Bertram .25-25 brass.  It's better than nothing, I guess.  It comes in a number of different lengths, even in the same box, but  all of them are shorter than the design length.  The heads and rims are frequently smaller than specified as well, and the rims are not infrequently oval.  You may need to experiment with different shell holders.

They do, however, fit in my rifle, function and extract OK.  Accuracy is not as good as any of my .25-20's or my .25-21.  I don't think that is because my rifle is inaccurate or my reloading technique is bad, but what do I know? .

I anneal the brass as far down as I can short of the head area to reduce case failures.  Even so, they not infrequentlly split after a few to a half dozen firings.  The splits are always far enough down the body to preclude trimming the cases and using them as .25-21's.  However, the splits have not leaked gas back or anything catastrophic.

I haven't tried Rocky Mountain's .25-25's. but I do use their .22-15 Single Shot cases, and their .25-25's probably behave in a like manner.  The RMC cases are very precisely turned from solid, so the dimensions are right.  However, if I reload them like regular shells, i.e., full length resizing, expanding, etc., the necks will split after a few firings.  It's best to use these as "Everlasting" cases:  never resize, seat bullet in case by hand and use black powder.

Thanks guys.

The action is a Stevens 44. From what I understand, the 25-25 was made specifically for the model 44 when the rifle was first introduced. It is the only one I have seen for sale in that caliber and I thought it would be interesting to to try out. I doubt there are many left out there, and even fewer that still get shot. It will be fun experimenting trying to find the right load. I will start with 25-21 powder specs because I hear it doesn't foul up the barrel as much as using 25 grains.

I too have one of the 44's in 25/25, a 44-45 with a perfect bore.  When I got it many years ago Bertram was all I could find for brass.  
I breech seat a 86 gn. bullet with a homemade seater and have never sized a case since the first firing.  I hold a 4.5 gn. load of Unique in with a thin cardboard wad and load at the bench using a Pope copy of a de/re capper.
Very pleasant for a afternoon shooting, accuracy is better than I can hold and I have yet to have even a single case failure.
Bertram might not be the best looking brass out there and for hotter loads it should be avoided.  The 25-20 SS Bertram in my 2R Lovell just doesn't stand up at all.   For the lighter loads ( and that is all you should use in that 44 anyway ) is seems to be a good source of hard to find brass.

I learned my lesson using Bertram brass. I'd go with RMC brass, but be sure to anneal it prior to reloading.

Loading blocks for the 223 / 222 / 5.56 mm US ought to work just fine.  Shell holder might also work.

Yes, the .223 is a close fit, and I've heard some people make their brass from reformed .223 also. It involve turning the cases on a lathe, and sounded like too much work to me, so have never tried it.