Find latest replacement_id in a table

I have a table with numbers in columns named supplier_id and supplier_replacement_id.
The supplier_replacement_id can appear again in the same table as supplier_id.
How can I find the latest supplier_replacement_id that doesn’t have a corresponding supplier_id?

Supplier_id → Supplier_replacement_id

A → C
C → G
G →
How to get G.

See this answer: Building Org Structure - Parent_Child Structure - #8 by dgudkov