Hi, If you've found this info, please ignore my posting.
There are several reasons why an obituary can't be found; one wasn't published, the newspaper of the time didn't microfilm their newspapers and/or turn the microfilm over to the State or a local library, an error in the death index, etc.
You didn't mention who the obit volunteers were. Did you try RAndom Acts Genealogical Kindness (RAOGK)? Did you try the Sacramento County PUblic Library?
There is a volunteer at RAOGK who will do look ups at the Sacramento County Recorder's Office where all birth, death and marriage documents are kept. Go to raogk.org/, read guidelines, choose California, choose Sacramento County and choose Wallace.
You can also request her death cert from Sacramento County Clerk/Recorder, ccr.saccounty.net/ . Successful Searching!