I'd try a brickyard since what you need to replace is terra cotta. It might help you greatly if you can either take the old one with you or take a pic of it with you when you go shopping for one.
We had downspouts that went into terra cotta piping and that piping went into the ground. The pipes were filled with all sorts of stuff. Instead of trying to keep it clean we are going the rain barrel route.

Have you thought of trying a patching compound like the stuff to fix broken concrete--it might work. I think it comes in small tubs at the box stores, a vinyl product?

We were pretty sure our terra cotta underground piping was failing - So disconnected them from downspouts & did corrugated plastic type above ground to the downspouts - & away from the house. Much better system.
I sure the terra cotta was filled with all sorts of things underground