Check out eBay or your local Buy & Sell newsgroups/newspaper.  You can find
new / returns / liquidated t18z package with phone, battery and charger for
about U$10 on eBay.  OR try the do-it-yourself solution:

Buy:

  - 4 "AAA" rechargeable NiMH batteries rated at 750 mAH or higher
  - 1 battery holder for 4 AAA cells

Take a slotted screwdriver and carefully pry open the case.  Remove old
batteries. Take plier and pull apart the battery pack and keep only the
piece that makes contact with the phone.  Note of the correct orientation of
that piece.  Clean out the adhesive that held the batteries.  You'll notice
the batteries used to make the battery pack are "AAAL" or "L-AAA" batteries.
These batteries can cost up to 15$ a piece if you can find them.

Remove the springs from the battery holder and glue that to the T18z's
battery holder.  Wired the battery holder to the contact piece and solder in
place.  Insert the new "AAA" batteries.

Now you have a phone that uses "AAA" batteries.  NiMH batteries are needed
to power the phone for eqivalent talk-time as old batteries.  But high
quality "AAA" alkaline/lithium batteries can be used to power phone for
several minues if need to.

I don't suggest buying the "Canadian manufactured" or "newly manufactured"
batteries for the T18, they're usually made using the above method, but with
low quality "AAA" batteries welded together.  They don't match the
talk/standby time of the OEM battery pack.  I bought one of these on eBay
and felt ripped off because I could've made one using better quality 800 mAH
batteries.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3045566076&category=20344

I got one of these last week...great battery.