What is the field for grabbing Bond Amount Outstanding by get_data()?
Besides, how can I get the bond rating by date specificly?
To find any such fields, please use the Data Item Browser (DIB). A very good example of how one may use the DIB is shown in my colleague's article. As an example, I looked on the DIB for a bond (345370BR0) and searched for 'Outstanding', and found the field 'TR.CA.AmtOutstanding':
For ratings, I would point you towards this question where I talk about bond ratings in detail - I talk about the dates of ratings in the answer provided.