These doses are based on averages given in the literature for doses WELL UNDER the doses that caused GI upset. All three compounds are GRAS and the resveratrol was not associated with ANY of the compounds in grapes that cause problems in canines. At all.
The chart has “doses” that are based on the ability to find these things on places like Amazon.
So you would just search “Resveratrol 250mg” on Amazon to find that. It's not a common size. But it's there.