Any ideas on where to get yellow powergrips online? nowhere seems to have them in stock, even the company itself.

maybe they don't exist (anymore)

I searched for yellow ones for a while, but gave up and settled for black. Try calling around to some LBS's, I was able to find blue, purple, and some other colors that apparently had been laying around for years. Most shops were surprised to hear me asking for them.

Some dude on here recently got a yellow pair from the company's website, but he must have gotten the last pair. Give them a call though, can't hurt.