You can find a specific commodity by using the search options within the cargo window. Select the search section and then find the section called commodities to mine. Using the select box you can choose the commodity you are searching for.
The search will only show commodities on planets your faction have discovered, and which have mining enabled and no obstructions such as planetary shields.
If you are in a system you can see a list of all commodities by interacting with the system commodities group within the engineering window under mining as well.