If your fob is the style jij posted above (my 2002's is) you can get a replacement case from Autozone made by Dorman.
They don't make one for the 4Runner (that I can find on their website -
www.MyKeylessRemote.com) but they do make one for a 2002-2006 Camry that is identical except for the small inner ridge on the bottom of the fob. Part # 13614, it's $15.
You will need to Dremel away the ridge on the inside back cover where the battery holder is attached to the circuit board (it will be obvious to you once you put the circuit board in the new back cover) but that only takes about 5 seconds.
They even had the button battery (CR 2016) so in 3 minutes I was all set with a brand new fob.
I got this tip from Mountain in another thread but tought I'd post here as so many are asking.