None of the big guys here in America makes brass for it. RWS makes it but I have found only a few boxes of it over the years- excellent stuff if you find any. Norma and S&B sell tons of ammo, but no brass that I'm aware of. (As for shooting that stuff to get the brass, beware that it's loaded hot and that accounts for two strikes against its potential case life.) Sometimes old U.S. brass turns up- Rem-UMC, Western, Winchester, etc. and it's generally good stuff, but take care to anneal the neck/shoulders of it to prevent splitting, it's old and a bit brittle, and hold your loads down to a sane level. I use 60-70 year old brass all the time, but I don't hotrod my .22HP's either. Ditto old stocks of Canadian made Dominion stuff.
Truth be told, your best bet is to just use .25-35 brass (available once again, but for how long is anybody's guess). One pass into the .22HP die and your all set. .30-30 is another option and it yields excellent brass but you'll have to go through some gymnastics to get it down to .22HP. Because .30-30 brass is a viable source there will never be an excuse not to load for the HP (bullet availability not withstanding- get a mold and never worry about bullets ever again).
