Definitely Doctored Locks:
Or if you live in Europe and happen to need veeeery special colors (instead of dyeing the extensions), shop at Plasikhaar:
They're much more expensive than Dr. Locks though, even considering postage & customs fees.
I haven't tried UV white yet, I believe it should look perfectly normal in daylight though. UV only means it's blacklight reactive. What you shouldn't do is mix different shades of white (like snow white and silvery white) or even the same color bought at different stores or from the same store at different times, as the dye / processing may vary slightly from lot to lot.
If you need to match the extensions to a base wig you have, and don't know the color number, the safest method would be to send in a fiber swatch to Dr. Locks and ask them to find matching extensions for you. If you're making an entire wig from scratch, however, you can blend the different extension fibers when you make the wefts, so you may not have to worry about the exact shade as long as the color blend is distributed equally over the whole wig.