I've had the Tone Monday brake light flasher for 9-10 years and I still love it. You can only get his version if someone sells, so forget that.
I just Googled "third brake light flasher" and a ton of stuff came up. Just make sure the one you get can be adjusted two ways like Tone's. For the duration of the flashing and the speed of the flashing. I have mine set for the shortest duration and a medium flashing speed. I think it flashes 7-8 times in ~3 seconds.
Don't get one that flashes constantly the whole time you're on the brake pedal. That would be ultra annoying to other drivers and not cool at all.
I almost forgot I have an old video of my third brake light flashing in action on my website. Same vid, two file types-
http://www.drivehq.com/file/df.aspx/publish/mgpa/vids/brklt.wmv
http://www.drivehq.com/file/df.aspx/publish/mgpa/vids/brk.mpg
Mark Griffin