Where can I buy a pipe for a Weckmann in the US?
Doing a quick Google Search "clay pipes for Weckmann", the only shop that came up immediately is The Pipe Shoppe.com. They seem to import them in various sizes and they sell them individually. You can contact them by email if you want to buy several, presumably they may be able to give you a deal on shipping and maybe a better price if you need dozens of them... though they seem to only have 12 in stock right now in this smallest size.

The only other option I could think of is if you happen to know someone in Europe who could order them from Amazon and then ship them to you.