I love my tie-dye lab coat! It's fun and cheerful and makes me happy. And the students love it too - it certainly gets their attention!
It's a little different from these - it starts with red at the shoulders and works its way through the spectrum until it reaches violet at the hem. Mine came from Flinn Scientific, who will only accept orders from "certified science teachers" and only ships to a "certified school address". If that doesn't work for you, check out the first link for Amazon, who will ship to anyone!
I was fortunate enough to receive my lab coat at no cost, as part of a program I participated in. However, I realize they can be a bit pricey (though I will say, it is a heavy duty coat, much thicker and heavier than any others I've encountered). If you love the look, but consider them to be out of your price range (which is why I hadn't bought one on my own), consider a plain white lab coat and some of your own dye. You could even use the chromatography pinwheels to make it a truly scientific lab coat!