:welcome:
The lamp assemblies your M9xx series uses is the MN10 LOLA(Low Output) and MN11 HOLA(High Output). You can purchase these at any Surefire dealer and perhaps at your local outdoors/sporting goods stores who carry the Surefire line. CPFMarketPlace always has some people selling off their lamps, might be also a better deal to get it from someone there.
For upgrades, there is the Surefire KL6 LED head(a more flood and runtime oriented option), which is discontinued and the upcoming, not released yet KL9, which I'm looking very forward too.
There is the option of getting the Surefire KT4 turbohead(second picture in your post) to run the same lamps but with a more concentrated beam profile for distance visibility. I have tried this setup and find it a little on the heavy side for balance on a M4gery+RIS/RAS setup.
There are some other options which take more thought, planning, and funds, such as Fivemega's MN series bi-pin lamp socket, which allows for hotwire lamps to be used for greater output. Also some people have discovered the LumenFactory D36 LED drop ins will work in this bezel, but requires some disassembly. That's all I can think of now. Should be a couple more options.
What color LEDs is on your M910a?