It looks like the trouble you're having is that the only CA Death Indexes available online are only for 1940-1997. That's annoying since LA county's death certificates go back to 1877.
This is the place you'd need to get an informational death certificate copy from:
Birth, Death & Marriage Records - the trouble is that I believe you have to know the date of death in order to request the record. What you need is access to the index which they apparently only provide in person. At least, that is the impression I get from reading that.
Do you know what religion she was? You could try churches in LA which may have cemeteries or burial records or they could at least pinpoint the area she lived in and you could look in non-denominational cemeteries around that area. I tried the LA directories on Ancestry.com but didn't find anything.
Try to confirm whether she went to LA to be with one of her children. Do you know that any of them did indeed move there? If you can find a record of one of them in LA, that will give you a starting point. For example, if you find a directory record of one of her kids in LA, you can then look in that area of the city for cemeteries.
Also, be aware that Nancy can be a nickname for names like Anna and Agnes.
Lastly, having said all that, keep in mind that newspaper articles can be inaccurate. Searching LA may be completely the wrong direction. Apply all of this to Routt County, CO too.