Mapping files are available on the GO website but these files contain cross references other classification systems like KEGG or EC numbers.

If you're looking to retrieve GO annotations for your organism based on UniProt identifiers, there are several ways. For a non-model organism, you could use QuickGO. See also why QuickGO and AmiGO may differ.

1. Go to QuickGO and
2. View Annotations.
3. In the Gene Products dropdown, add your UniProt IDs and Apply. You can download the GO using the Export button.

You can also grab GO information on UniProt

Remember to check the GO citation policy and license if including GO in any publications or other work.