Personally, I wouldn't bother with whitewalls. I had them on one of my Harleys (before I saw the light and switched to Triumph!). They are a pig to keep clean, especially the rear. And that was with belt drive. I would think it's ten times worse with a chain.
If you must use them, I'm trying to remember where I got them. I think it was from a guy in whitehaven who advertises on Ebay under the name japscrap. They were certainly cheaper than the stealership's blackwalls. I think they were Avon. Cost me a tenner to get them fitted but now I've got tyre levers!