Believe it or not, we just got back from a wedding in Detroit this weekend where my 3 year old was the flower girl. In preparing for the wedding I searched everywhere for something appropriate for our 18 month old son to wear and could not find anything! It was impossible to find a toddler sized tux or suit, let alone a nice outfit other than the standard khakis and chckered shirt. I also searched everywhere for dressy shoes - totally impossible to find in any of the standard shoe stores at this time of the year, I guess. Stride Rite does carry the nice dressy shoes and I do like Stride Rite usually - but again, couldn't really justify the expense for shoes he wouldn't wear often. I did find some dressier type "Fall" shoes on clearance at Gymboree that could pass for dress shoes and still be worn this Fall. There is a kids clothing store in Geneva Commons in Geneva that sells really nice dressy outfits for little kids - but its very expensive and I couldn't justify the cost for something that he would wear one time. However, I did find a store at Stratford Square Mall in Bloomingdale which I would highly recommend. It is called something like "Wave Kids" - or something like that. Its a store for kids ages 0 to maybe 7 or 8 and its all dressy outfits. They have tuxes and little suits for boys starting at age 3 months. And the selection was excellent. The owner of the store was there when I went and she was very helpful and said that if I couldn't find the outfit I wanted in my son's size she could order it and have it delivered within a couple weeks. I bought a cute little beige "tux-style" outfit - pants, shirt, vest and little bow tie for only $36. They also had more traditional style tuxes with vest and jacket as well as just dressy suits (pants, shirt, jacket and tie). They also have dressy shoes that match.
Not sure where you live, but I'd recommend just going straight Wave Kids at Stratford Square Mall and avoiding the headache that I had searching before I found that store.
Good luck!
K.