I’ve been using these, you can use 1,2 or 3 AA cells, and the back connector works properly with a Maglite, unlike the 2 cell ones. http://www.ebay.com/itm/272508350062
I asked the same question when I bought my Fenix TK-70 a few years ago.
I ended up buying Tenergy Centura 8000mAh D cells. They are good quality and low self discharge. I bought them either from Amazon or from a seller on ebay (I can’t remember)…
I have both Powerex Imedion and Tenergy Centura LSD cells bought for my Fenix TK70s back then. I wasted the extra money buying the much more expensive Imedions for the Centuras retain their capacities much higher over a period of time over the Imedions.
I don’t know if there is connection here, but I also bought 16 Imedion AA cells for my Fenix TK41 during that time and all have them are now junk due to leakage and poor capacity retention.
I can’t remember exactly where I got mine (I think it was one of the Chinese websites, maybe Fasttech?) but they look just like these. They’re parallel so you can run them with only a single AA Eneloop loaded up if you want, or add more for longer runtime. And yes they work great in series applications like longer Maglites.
I like the Tenergy Centura cells, too. Good sales don’t happen often, though. I got my D cells through Newegg’s marketplace a few years ago at a great price.