With my Nikons I use the Nikon SD-9 battery pack. I fold the flaps on the pouch over my camera strap, and that way attach it to my camera. I use NiMH rechargeables with these.
With my Canons I use the Canon CP-E4 battery pack, and I clip this onto the camera strap as well. Alternately, with the flash bracket that I use, I can screw the battery pack onto the vertical stem of the flash bracket. I also use NiMH rechargeables with these battery packs.
Quantum Turbo SC battery pack - (SC = Slim & Compact)
For more juice than the battery packs with rechargeable batteries, I use the Quantum SC battery pack. As the name says, it is slim and compact – yet offers great capacity.
If I want more power than that, or need to fire the strobe faster than the packs with the AA rechargeable batteries can deliver, I use the Quantum 2×2 Turbo battery pack clipped onto my belt. It is heavier and more bulky than the Quantum SC, but sometimes this is what is necessary.
Check this page for details about the specific connector cable for either the Nikon or the Canon speedlights.
The advantage of the Nikon SD-8A and Canon CP-E3 over the Turbo 2×2 or SC, is that by clipping the much lighter battery pack to the camera strap, it leaves the camera bodies independent of me. That makes the cameras easier to set down, or switch them to the other shoulder.
Another advantage over the smaller battery packs by Nikon and Canon, is that they won’t damage your speedlight when you rapidly fire your flash. The Quantum has the potential of doing that if you’re not careful.