I have an Excel containing cell comments/notes, I've uploaded it to Google Drive, and converted it into a Google Spreadsheet. How may I retrieve the comments for cell A1?
The API doesn't describe how to get a note or comment from a cell.
The Google Apps Spreadsheet API only provides programmatic access to spreadsheet data. You cannot access comments using the API.
You can get the Notes for a cell or range of cells using Google Apps Script. See the Class Range documentation for these methods:
There are open issues related to this functionality, mainly concerning comments (the "other" type of note). See Issue 1818 and Issue 2566
In a comment on your question, SGC asks if you've looked at developers.google.com/drive/v2/reference/comments/get and developers.google.com/drive/v2/reference/comments/list; these are FILE level comments, not the comments attached to a spreadsheet cell.