20W-40 is most commonly a motorcycle oil, beyond that it's kind of an oddball. Did you try a motorcycle shop?
I know AMSOIL also makes it in synthetic, others may also.
If you're mostly using it in a warm climate, I wouldn't think 15W-40 would make much difference, it's only a slightly thinner base.
Also probably a heck of a lot cheaper since it's readily available most anywhere.