Alro Metals has a place here in Sarasota, or at least they used to. Haven’t checked on them lately, but have bought millions of dollars worth when I worked at Hynautic. They supply to industry, but will also do a retail piece.
I found Onlinemetals.com, 6 locations around the country, probably saved on shipping only having to come from Hotlanta, 15% off online today. Get alum. 6061 T6 if you want to strengthen your transom, I called them and that’s what they said to get.
I’ve had better luck in reinforcing F’Craft transoms by changing the transom knees to cast pieces. G3 boats have a cast knee on their smaller models which fit on my ’57 Ranger. Two of these replaced the fabricated sheet metal original. Added more strength than a large flat piece.