I agree there are more bullets available in .338 than .358, but there are a good many bullets available for the .35's. If one cannot find a suitable bullet in .358, they're being extremely picky.
If your not put off be a wildcat, how about the 375 Whelen. Brass is easy to make, no shortage of bullets and would be a great match up for the Enfield.
A .375 Whelen is another one to consider for sure.