Nine foot oars are rather unusual. There are several places to buy them but none are cheap. It also depends on the kind of boat. do a search on some of the boat building sites. Try bateau.com I puchase boat plans from them and they have sources for just about everything related. The only other option (if you survive the sticker shock) is to make them. Plans are available free if you search the right places.
I love boats it's building them I can't stand.