It's longer than the .303 British it's based on, otherwise you could just use those cases (2.22 vs. 2.5" case lengths).
QualCart doesn't list them but they may have expertise. I guess my next question is are there other, longer cases based on the .303 dimensions that might serve? Chuck Hawks says that cases can be made from .405 Winchester. (If you need .405 cases I have a crap ton of them.) I will say that there is some mojo and variability in the rim thickness on .405 cases causing headspace and action closure issues, and this might be a problem for your particular rifle - Sharps (they're commonly used for .40-70) and Winchester people seem to have issues and problems with this. Might have to file case rims down or build things up somehow, especially if yours is a British double based on who knows what original standard.
CH makes a full suite of dies.
https://www.chtoolanddie.com/calibers/caliber?id=375Fla2.5Lastly -
https://beaksandbrass.com/product/375-nitro-2-1-2/ $6 each