Most 3xCR123 lights will take 2x17500. Keeps the voltages around the same place. Most of those probably won't take an 18500.
Some larger AA lights (The BLF special comes to mind) can be bored out to take a 17500. I don't think any AA light is "fat" enough to be bored out for an 18500.
I don't know of any single cell 18500 lights - it is not a common form factor.
I have just had a closer look at mine and realise it is a Lego job. The tailcap is from one of the fake Solarforce bosies as sold by Lightake. Everything screw fully home with an 18490.
Peak LED Solutions makes some single cell 18500 models. Most models are available in brass, aluminum and stainless steel and some in titanium. Robyn and Curt make some lovely toys, but be warned that only the smaller models like the Matterhorn could be considered budget lights. Lots of flashlight porn to be had there though!