My shower head is leaking and the hot and cold knobs are closed. I took the 1/2 union off the hot and cold. What can I use to plug the hot and cold pipes?

Are you intending on eliminating the faucet? Is the pipe copper?

I am not sure what you are trying to do so I am just going to guess that you are trying to cap off the waer going to the shower faucet. You said that you took off the half unions, if you took them off a threaded pipe then go buy a couple of 1/2 inch pipe caps and some teflon tape. Wrap the male threads with the tape (about three to six wraps) depending on the thickness of the tape. Now screw the caps onto the threads and tighten with a wrench. It does not have to be real tight, the teflon will seal the threads.

If this is not your goal post again and be a little more specific as to what you are trying to do.