I found her in the 1940 US Census at Familysearch; here is that link:
https://www.familysearch.org/ark:/61903/1:1:V1T2-5B9
I also found a transcript of her obituary at Findagrave, in which her first marriage (to the father of her children) is not mentioned. Here is that link:
https://www.findagrave.com/memorial/67651319/isabel-miller
It should be noted that everything I've found on her (so far) has her listed as Isabel or Isabelle, not Margaret.
Do you by chance have a library card at a library that subscribes to the library version of Ancestry? At the moment, my hometown library has made it accessible online (when there isn't a pandemic going on, it's only accessible from the library premises). You can check with your library's reference desk to see if they have this resource, and if so, how you can access it. I asked this question because I found some records on this person that way. For instance, she is mentioned by name on the marriage certificate of her son, and I found a copy of that document at Familysearch. Here is a link to that:
https://www.familysearch.org/ark:/61903/3:1:3QS7-L93W-NSMN-1?i=1219&cc=1804002
Anyway... the above should be enough to get you started. :)
