Yup,
Brass is available from bertram and Rocky Mountain Cartridge. I have both and prefer the Rocky Mountain stuff. It is more expensive but is custom made and should last a LONG time if taken care of. I have had some of the bertram stuff split at the mouth upon first or second firing. I have read that once anealed it reduces this tendency.
Dies are available, Someone else please chime in, but I think it is CHD4 and I want to say that RCBS makes them. I got mine from Huntington's in West Virginia. I would think they or Buffalo Arms can help you out with dies and brass. But check out the Rocky Mt brass, good stuff.
I have a 44.5 and it was a 25-20ss. I also had a 38-55 barrel put on by Rocky Mt. Nice thing is you can switch barrels on the 44.5 if you are so inclined. I really like my 44.5 but have had problems with the firing pin and the trigger. Firing pin is a common problem on the gun but can be had from CPA for a reasonable tarrif. My trigger had been monkied on by several people over the years and finally got below the original hardeding on me about half way through a match. I don't think the trigger is a common problem on the gun.
MerwinBray 

BTW Midway has the bertram listed in my last catalog

Buggybuilder,
I wanted to ad, to me, one of the things that is so darn fun about a classic gun and a classic cartridge is the classic accesories that you can find to go with it! Re and de cappers, old molds, old tools, etc. I enjoy the tools just as much as the guns. Don't rule this out as a possible source for loading. One of my prized ideal tools is my 25-20ss tool with mold. It throws a good bullet that was designed pretty much for that gun, and loads nice to boot, if you are going fixed ammo. Mine is in nice shape, not perfect exterior but good mechanically, so I can cast and load with it and not worry about messing up the nickel.
Lots of other 25-20ss out there, it was right popular for a while.
Merwin

I've got Stevens in .25-20SS, and also .25-21 Stevens. Love them both, but the .25-21 is my favorite of the two.
Bertram brass is junk! Go for RMC brass, as it's well worth the extra money!
I got dies from Buffalo Arms, C&H 4D for around $62. Might be more now, but still cheaper than most other brands.

  I'll 2nd most of what's been said. Don't get Bertram brass, as like mentioned, they split out, and at the price per case a 20% reject rate hits you in the pocketbook fast. I even tried annealing them and didn't find any improvement. It seems others have had a different experence so it might be the lot I got.

  I make my own cases now and haven't had a bit of trouble with them.

  My experience with CH4D dies has not been good. The two sets I've gotten both scratched the sides of the cases. Don't know if they would have weakened the case any but sure was unsightly, and after sending a coupla cases to BA.... where I got them from...... they refunded my money and applied it to a set of RCBS's. Like everything else I guess you get what you pay for.

PETE

Joe,

  That's why I make my own now. Altho I can't say they won't fail since I'm working on my first case yet. Got at least 200 shots thru it. Did have three primer pockets come loose from pushing the envelope.   

PETE

I cast a 104 gr. for both my Stevens .25's, and it shoots great.

Most of the older model Stevens .25's had 14 inch twist and will stabilize the 86 grain bullets ok with a reasonalble charge. The later Stevens .25's had a  13 inch twist which will stabilize the 100 grain bullet. Also available on special order was the 12 inch twist which, I have heard, was sometimes used with .25-25's. From what I've observed factory Stevens were not available in the .25-35 or .25-36 which sometimes used faster twist. Of course anything is possible with old single shots so maybe you should check the twist.