I see no one answered this question; I purchased a kit off of eBay for my Altima. I paid about $14 for it. Luckily my car was black so I just bought the black kit. It worked pretty well, and was fairly easy to install, the hard part was drilling holes in my bumper. Not that it was physically difficult, but emotionally difficult, I measure about 10 times before putting the hole saw to it. :laugh:
I'm not going to post a link to an ebay item since the ads change and it'll just be a dead link soon, but if you ebay search "parking sensor kit" you'll find them dirt cheap.