Hi Liz, I found the Cow Head Ranch Family Tree, trees.ancestry.com/tree/6689097/person/-862998488 which shows that James A. Casner was born Dec. 1839 in Missouri and died June 1890 presumably in Umatilla Co., OR.
I didn't find anything on-line about his wives. I'd contact the Oregon State Archives for Linn County, arcweb.sos.state.or.us/pages/records/local/county/linn/inv/index... They have marriage records before and after his marriage to Clementine. Hopefully the marriage record will contain information about her.
The Linn Genealogical Society, www.lgsoregon.org/ also might have some info about James and Clementine.
The Oregon Genealogical Society, oregongenealogicalsociety.org/ is another organization that does research.
Marriage records can be found at the Oregon State Archives, arcweb.sos.state.or.us/county/cpgrant/marr.html .
The Grant County Genealogical Society also does reserach. Contact Linda Cook at PO Box 418, Canyon City, Oregon, 97820.
There is an obituary volunteer in Umatilla Co. at Random Acts Genealogical Kindness. Go to raogk.org Click on Oregon Click on N thru Z Scroll down to Umatilla Co. and choose Jessica.
If she won't check an entire month's worth of obits, then go to the Oregon State Archives Umatilla Co, arcweb.sos.state.or.us/county/cpumatilladescrip.html to see if they have death certificates for 1890.
If was living alone by then, an obituary might not have been published.
You might want to contact the Blue Mountain Genealogy Society, rootsweb.ancestry.com/~orbmgs/. They do research and are located in Umatilla Co. Successful Searching!