I don't seem to be fully comprehending posts this week when I read them.
Check out both Ozark Mtn Mini Tack (im not seeing carts anymore on their site) and
Mini Express Tack - neither shows a single tree by itself BUT they can get the carts and I bet they can source a singletree for you!
I deal with these guys a lot, but don't know how close/far you are from them. They take orders over the phone and can also do CC/DB cards over the phone but they are Amish and while they now have a phone in their shop, it's not answered all the time. THEY ARE GREAT!! They will also do partial refurbishing of other carts brought to them, and complete overhauls of carts as well as building their own styles from "scratch" and can do some customization to those individual styles. Their prices are VERY reasonable - including shipping - and fast! They have a catalog and also specialize in Bio & Beta harness both pleasure and work styles.
Fairview Country Sales
Tack Shop
Address: 3062 Co Rd 160, Millersburg, OH 44654
Phone
330) 359-1501
I'm sure there are others that may be in your area - not sure where you are. Rural Heritage - both the website and the magazine will also have advertisers that do the singletrees w/ the different ends...
Iowa Valley Carriage carries the whole single tree if needed. Also, google Driving Essentials and see what they have. There are a couple of other companies, not thinking of them this morning...