I'm sorry I can't help you in regards to finding any stores in or around Pittsburgh that might sell the Pittsburgh Steelers baby caps. I tried doing a search on this site in the event there might be a thread listing stores in Pittsburgh that sell Steelers merchandise, but the search function is not working for me. If you are in the Pittsburgh area, you may want to look in the yellow pages for Pittsburgh Steelers gift shops.
I did find a couple online sites that I believe still have them in stock and sent those links to you via PM. Hope they are of some help to you.
How exciting that you and your wife are expecting a lil Steelers fan any moment now. Congratulations!