Lumens Factory makes some replacement lamps. I don't know if they're as bright (doubt it), but you can run some of them on a pair of 17670's if you want a rechargeable option. They also make a lamp specifically for the M4 that runs on primaries, but I don't know anything about it.
Another option is one of
fivemega's Bi-pin sockets, and you can run whatever bulbs you want.
LED tower modules show up sporadically. yclo offered up a handful with Seoul emitters earlier this year, but it looks like he sold the last one back in August. AW appears to still have the
kits available if you can build one, or know someone who can.
