Where can I get 13by 13 Terracotta flue?

[Dmitry]

I want to extend my terracotta flue before getting the new liner in. It's 13 by 13. I have three flues and the middle should be extended or cut to fit certain top plate I like.
I did a quick online search and didn't see anything on sale. is there some specializing local store I should be looking for?
Thanks.

 

[begreen]

Masonry supply store

 

[Dmitry]

Masonry supply store

This is my current setup. I want to use the same top plate as on the right. Do you think adding one more terracotta tile would be feasible in this situation? As a side benefit that will extend my liner that's on the short side. Thanks.