Could it be . . . lemons?

Fresh lemons are still alive, and they have active
defenses against oxidation of their bioflavinoids.

Once you separate the bioflavinoids from the lemons,
they are attacked by oxygen from air.  If you want
a concentrated lemon bioflavinoid preparation,
you could make it yourself by stripping lemon peel
from a fresh lemon and extracting them with a
suitable solvent, such as ethanol.

I'll suggest several things. Consider the bulk powder supplements
for quercetin and rutin as flavonoid sources. And as Mark suggests
lemon themselves
might be a way to go. Be sure to get unfumigated lemons or citrus if
you eat the skin. The antifungal chemicals used on most citrus are
rather
undesirable for consumption. And even when one peels the skin,
one wil get a dose as the knife will spread the fungicide around.
I'd also consider eating the zest as well as the rest of
the lemon in an organic class lemon. That is to say the whole lemon
skin and all.
Well maybe not the seeds.

Original French Pine Bark Extract contains over 40 types of flavonoids
Getting 1 type of flavonoid is quite controversial (quote Dr Lester
Pecker)