I had a difficult time finding a suitable replacement in the U.S.
I tried the 12V 30Ah Duracell Ultra 400CCA battery mentioned earlier in this thread, but at 5.125" wide it was slightly too wide for the M3W battery tray.
Today, I was able to find a
12V 30Ah Xtreme AGM 385CCA Battery Item Number CYLA30LXTA at my local Batteries Plus and it was a perfect fit at 4.92" wide.