USPS.com
Not plain, but the popular ones that are pretty good are here. The site is already burnt out, so I don't want anyone bitching about, "OMG YOU IDIOT."
I personally will start buying mine and searching for better ones so they don't peel off. I don't know about other areas, but I personally think that slaps are a big thing here in L.A for the main reason being that they can last longer than anything else since they don't get buffed as often, depending on the street.