Data Set/Database listing NAICS codes for U.S. firms? I need access to a dataset that matches firms to their NAICS codes so that I can fill in some missing data in a data set I have for some research.

I was wondering if anyone on here might know of such a database or dataset I could explore?
If NAICS are not provided in the dataset, you need a second variable, related to NAICS, to do the match. Two options are ISIC and SIC, related to industry classification. You can find the official link between these two and NAICS here. These are Excel files with the concordances, which will not be perfect (sometimes you have m-to-1 or 1-to-m matches). There are more crosswalks between classifications here, here, and here.

In terms of Stata files (if you need that), check file [C1] here. There is also this very good NBER crosswalks (under Concordances heading), where you get SIC-NAICS crosswalks in Stata, SAS, Excel and CSV formats.

For reference, here you can find a mapping between SOC (occupations) and NAICS, but I am do not sure how it works, as I cannot see a 1-to-1 mapping between occupation and industry.

Finally, here there is a document describing some technical aspects of the crosswalks.