Correct way to say "by itself"?

You would say:

I didn’t get those shoes because they were pretty bulky. The pair I got are pretty bulky themselves.

You clearly got these shoes not those shoes.

Similarly, you might say:

I’ve misplaced a pair of my favorite shoes. I wonder where they are.

or

No thanks, I already have a pair of shoes.

Great, I can’t wait to see them!

You wouldn’t say that you couldn’t wait to see “it”, because most people would not call a pair of shoes an it in these circumstances. Here they’re a they. It’s how couple can be construed as either singular or plural, depending on the circumstances and the speaker’s intent.