Iowa Valley is more of a harness supply you need a carriage supply.
Whittmer's Coach Shop, in New Holland, PA is my favorite, but they don't have a web site.
You can call and they will send you a catalog for a few $, well worth it to have. Hook end Single Trees Horse Cart Parts Montana Singletree Parts Buggy Doubletrees Pony Single Trees
I haven't found any metal single trees, but here are some wood ones, You might have to order the piece that attaches the singletree to the cross bar too, cannot remember what that is called right now.
Can you take a photo of the way your singletree attaches to your crossbar?