I have an older Frigidaire cooktop stove with a crack across the surface. I replaced it once several years ago but can not find a source for the part. It is a Black top for a RB139CD electric cooktop. Can anyone help?
Gary
Hi,
You could post a wanted ad for one in the classified on our site at ApplianceJunk.com
Also be sure to search ebay, I have found some old hard to find part on there every now and then. 
Gary, I’ve checked a few variations of your model number and not one glass top is still available. Frigidaire had discontinued most parts for models made before 1979.
This is one of those things that local parts places would not stock. So finding someone that has one hidden in the back will probably prove unsuccessful.
Sorry about the bad news.
Good luck.