I was wondering which one of the following constructions below is grammatically correct? Or both are acceptable?
- Something to the rescue!
- Something for the rescue!
In the USA at least, "X to the rescue" is a phrase commonly used to imply that X is going to arrive on the scene and rectify the situation in some way.
I have never heard the phrase "for the rescue" before. There are perhaps some situations where it would make sense, but for the most part it just sounds weird to me. For example, it might be an intentional mashup with the phrase "For the win" (commonly heard in online gaming circles, along with a few other "for the ..." variants). Perhaps things are different with other English dialects, but in general American parlance that is almost certainly not the best way to say whatever it is you are trying to say.
In the specific example you give, "Something to the rescue!" is the correct one, if you are referring to the common ejaculation indicating something is about to engage in aid of the current situation.
That is not to say there are no situations in the English language in which the words "something for the rescue" is a valid combination.