Sure. The ‘d in “we’d” is a contraction of “had”. Use the first verb to form the tag. If it's a simple tense, then use "do" (does, did). We HAD better be going, HADn’t we?

It’s from the expression “had better...” which is used to indicate that an action should be done by the subject in a certain instance. In your sentence, without the contraction, it’s “... I suppose WE HAD better get going...” that’s why the tag question is “hadn’t we?”