Weird, someone broke into my 93 si, so i know that sucks,
Yet i've never heard of a thrid brake light being stolen.
EVER

Did they just steal the light? If so it sounds liek someone else with an eg, or someone who knows someone else with one.

If i buy a roof spoiler with a brake light i'd give you mine, but i wont know for a while.

If you have to try Honda Parts Deals for an OEM replacement, They have everything from trim pieces to rear fender panels.

Good luck!

Edit: Isn't the brake light on the putside anyway?