I typically buy Kona through equilter.com I prefer to get one brand because there is a difference from black to black, not to mention the various shades of things. While I find sometimes there are differences in the greige goods (the base stock of the fabric) in terms of subtle differences in weave or shine, I do find Kona keeps their colors true to each other so if I always buy Wheat or Sand or whatever, I can use the pieces from previous projects with new yardage along with my basic black and white I keep in stock. Usually buy a 3 yard cut of those when getting low.
I have a hard time finding Kona solids in shops and in my area they are priced quite a bit higher than through equilter. They are my first stop for fabric shopping on line, I've used them many times and have always had tremendous service.