I purchased several lens from flashlightlens.com, and they seem like decent lens. But they don't fit any of my flashlights properly.
For example, both my Dereelight CL1H and my Solarforce L2P come with 28.0mm lens. The 28.6mm lens was just a bit to big to fit, and while the 26.8mm lens will fit, it does not fit the gaskets properly. My Malkoff 2xAA comes with a 22.0mm lens. And both the 22.0 and the 22.61 from flashlightlens.com will fit, but they are too thick! Which means they will put more pressure on the sealing gasket than the brass malkoff module and may damage the gasket after a while.
In the end I have decided it's much better to just get a lens actually made for the flashlights for a more proper fit and seal.