The Bolt Cyclones are great battery packs, but you have several options on where to get them. Two of them are made by Godox (PB960 or PB820) and have been rebranded by Bolt (same as several other companies including Cheetahstand). Here is a very thorough review of the PB960 (same as Bolt DR PP-400DR): http://flashhavoc.com/godox-pb960-lithium-power-pack-review/
I have a pair of these that I mainly use with my CL-360 flashes (also a Godox product), but occasionally use with my Canon speedlight, too. This battery pack is much larger than the CP-E4 so keep that in mind.
I also have 4 PB820s that I was using before the 960 came out. These work well, too, but are NiMH, not Lithium batteries. This version was originally sold under Bolt, but that have since upgraded it to Li-ON as the Bolt PP-310. It's almost the exact same size as the PP-400DR but has less than half the power capacity. The only other downside is the lack of a lock on the cable to the camera. Having recently used one of these attached to my on-camera flash, I can tell you the cord lock is a nice and useful feature.
There is also a Bolt Cyclone X PP-600 -- this isn't from Godox, so I am not sure. It's an inch smaller on each side, which is quite appealing, but way overpriced for what you're getting.