The magazine for this battery pack (and apparently its successor, the CP-E4) is notorious for breaking easily. It's also handy to have a spare magazine for quick battery swaps. But according to B&H, the CPM-E3 magazine that fits the CP-E3 has been discontinued and is no longer available, and the CPM-E4 for the CP-E4 won't work.
I found one at Adorama, and some other places appear still to have a few, but it seems that when they're gone, they're gone.
