I found mine on Ebay and I love it. It's on duty with me every night.
I do have the LED SL20L, but it's 350 lumens, and it's disappointing Streamlight can't make a brighter version. They have a recent 450 lumen version, but it's not worth spending the cash for an extra 100 lumens.
I used to see the drop ins all the time on ebay and terralux was actively making them. Last time I saw them was a year ago at a gun store. I dont believe that Lumens Factory makes drop ins like they do for the stingers and other streamlight lines. Maybe with how many people still use them, if you email them they might make some?