神のみぞ知る = God only knows
のみ(only) is used quite commonly as in....
クラムチャウダー味のゲータレードは、ニューヨーク州でのみ販売されています。
Clam Chowder flavored Gatorade is being marketed only in the state of New York.
のみぞ with a ぞ attached, however, I have only heard used in the expression,
神のみぞ知る.
「クラムチャウダー味のゲータレードなんて、本当に売れるんでしょうかね。」
「どうだろうねぇ。神のみぞ知るってこういう事を言うんだろうな。」
" I wonder if Clam Chowder flavored Gatorade is really going to sell."
" It's hard to say. I guess this is what they mean when they say, God Only Knows."