Congrats on the new arrival!
I would try ebay and etsy.com. Just type in "U2" and then narrow your results by clothing, then by kids/infants. I don't know if I've seen any baby clothes with album covers, but I have seen onesies that say "I rock out to U2 with my daddy/mommy." You could also always make the shirt yourself, I know some photo-ordering websites will make clothing with photos on them.