The blinkie that I just tested from DealExtreme has a SuperFlash compatible bracket, and the whole blinkie is $3.54 shipped; it may be cheaper than just the bracket from Plant Bike. And the blinkie itself is very similar; not as bright directly on axis but brighter off axis, and the reflector works better than the SF.
Here's the review
and
here's the product page.
Note: Deal Extreme orders typically take 3 weeks to arrive. Don't order from there if you need the stuff quickly. Patience is a virtue.