I heard it's possible to modify a Game Gear battery pack for use in a TE. But those are getting very old and their NiCad based. Not exactly the type of battery you want these days.
I've never seen the official TE battery pack, who know it one was ever made, it would extremely rare. The GT battery pack is the only one I've ever seen.
Pretty much if you want a battery pack for your TE, you can use a third party one. Any NiMH type pack with a 7.2volts with the proper adaptor connector should do. The mah rating should be >2800 and that should give you 2 to 3 hours of game time.
I used to know of a place where you could buy a compatible battery pack for the TE, but that place stopped making them. If you know anything about making rechargeable battery packs you could go down the DIY route. Basically soldering together 6 rechargeable AAs into a battery pack.