The LX and M packs both show up on the auction site very occasionally. When and if you find one, you might want to know that it's possible to replace the dead NiCd cells with new NiMH cells. They're a tiny bit larger, so it's a tight fit. Another option for the the LX pack is to solder in the contacts for two standard rectangular 9v alkaline batteries. They'll fit, and will work - 18v is more than the 14.4v nominal of the NiCd battery pack, but a freshly charged pack will be up in that range at first anyway. You'd have to take the pack apart to replace the batteries, of course, so you'd want to cut the faux leather covering so it doesn't glue the case halves together.
An alternative is to look for the Battery Grip M and a Power Cord M. Then you'd have the choice of holding the camera using the vertical grip, or keeping the grip in your pocket (useful in cold weather). The grip uses 12 AA batteries (which is also 18v, so the above mentioned hack is obviously within spec.)
See also
http://kmp.pentaxians.eu/power/