Where can I buy the bottom(grinder side) of the arbor/adapter for cut off wheels. I have the screw on top side, need bottoms has no threads in it but has counter sink for top to screw into so wheel will be centered. Thanks

Hi,
Have you thought that maybe a brand and a model might help?
Peace,
Clarke
That info in hand, try eReplacement Parts at:
Repair Parts for All Major Brands - DIY Repair | eReplacementParts.com