It's probably a standard threaded Weller Tip.
You can order them online (try amazon) and I think you can find them at Home depot if you can believe that.
Anywhere they sell Weller products.. and if it's not a threaded variety of tip then it's just a piece of copper. Not easy to find a big piece of copper.. try online, amazon again.. they probably have that type too.
The other option (once you check the prices plus shipping) is that you just order a brand new Weller soldering iron. It's only a few dollars more with shipping.
my 2 cents.