They're not real easy to find. Usually if you find a nice one, it's in a nice car. You might wanna look around on your local craigslist and maybe make a wanted post for one and see what happens. I had one in my '88 GT and i loved it...it covered all my tools and junk in the trunk.
Mine came with the car when I bought it, I don't remember seeing any receipts for it so not sure where he got it or if the original one.
Good luck. Its a nice touch to help it look clean and complete. I've had a lot of people actually comment about it being in the car and the sunroof, both of which they said they don't see often.
It seems to be one of those things people remove and forget about, I wonder if it is to improve cargo capacity? This reminds me of how almost every Fox has the center console latch broken (I've been told original owners have broken these on the first day of ownership.)