after searching around I found two sets of whitewalls. One is the narrow set (which would be a more modern look, with a simulated 1"stripe) put out by pegasus hobbies, found on several sites, or the wide set (I guess an old style? which would fit this I think) can be found on detail master's website (http://www.detailmaster.com/Merchant2/merchant.mvc?Screen=PROD&Store_Code=detailmaster&Product_Code=PGH-1011&Category_Code=PGH direct link)
Most people of course say paint it on, but use acrylics. Thats ususally what I do and it ususally turns out pretty good if the tire was MEANT to be white wall, because it'll have a guide. Otherwise you could always make a round mask somehow