I'm trying to find a true white ready made buttercream
for a wedding cake. Has anyone used the CK brand
buttercream? Is there a readymade that anyone
really likes?

I have and was quite disappointed. I did, however, order it during summer months so perhaps that had an effect. I found it overly sweet and gritty.

If your buttercream has a yellow tint to it, you can add a dot (or two) of violet gel/paste color to it. It will cancel out the yellow & give you a white(r) icing.

Rae

I don't know anything about the CK brand, but H.C. Brill ready-to-use products are superior to any other commercial brands that I've ever heard of. You could try finding a local Brill supplier (if no luck, you could try the brand that Sam's Club carries).