I am a Catholic woman and just a couple of years older than you....I remember all the boys wearing the solid white little suits for their First Holy Communion. However, that was in the late 70's! We attend a LARGE Catholic church in SW Orlando, and my son had his FHC last year (and we just attended our friend's son's FHC last weekend)...the boys simply aren't wearing all white anymore. I truly do not want to offend you, and I can appreciate your traditionalism, but they are wearing any color suits (black, gray, navy, khaki...) with white shirts (that are doing the symbolic representation of this beautiful event) and, about half in colored ties/half in white ties. I can understand wanting to be traditional, but, in this case, I fear, if you do find an all-white suit, he'll look like he should be walking up the aisle to the beat of "Stayin' Alive"! Another beauty of wearing a colored suit, is that he will get to wear it again too! If you are still determined to do this, I often check E-Bay for specific items I can't find anywhere else (i.e. favorite Gap silk skirt, certain pair of shoes, etc.)
Hope this helps!