My SL3 also uses C-cell batteries, and that’s actually why I bought it. For some reason I found myself with a bunch of C batteries and not a single household flashlight that used them. So, when it was time to get a dive light…I use a Underwater Kinetics SL-4 eLED L-1 (I've had it for years now). It uses 4 C-cells, and the C-cells easily last for a one week trip (with use in daytime to highlight something, and for night dives). I don't know that they are easily available anymore.
I like the SL3, except for the twist on-off. Don’t like that it takes two hands.