Both are fine.
"I'd" is a contraction of "I would". Similarly "Would've" is a contaction of "would have". There is no way to combine all of these three words, so in each of the two examples, although they look a bit different they are almost the same. The difference is tense.
"If they hadn't found me, I would've been dead by now".
"I would have been" is conditional and in the past. The addition of "by now" only means that the death would have occurred before now.
"If they hadn't found me, I'd be dead now"
"I'd be dead now", or "I would be dead now" is conditional but in the present. In both sentences, "dead", is a present state, but follows a death, obviously. Although the conditional part of this statement is about the past ("if they hadn't found me"), the statement about being dead as a consequence is clearly in the present with the use of "now" (as opposed to "by now").