Can someone point the way to a good 21700 battery and a site that sells good but inexpensive batteries. I'm about to get a new mod that takes one 21700 battery. The only ones I have are a married pair I use in my Voopoo X217. If I break them up for the new mod I'll need at least a pair to replace them or maybe I should get four so I have a backup pair. Thanks for any help!