Best source is like Tex said...fresh, green veggies. But, cuttlebones/cuddlebones (depending upon the fid) are kind of a no harm/no foul based on my experience. I always looked at them as a beak groomer not as a source of calcium - same for mineral blocks.
ALTHOUGH... some of those flavored mineral blocks (veggie ones) came in quite handy with budgie fids reluctant to delve into delectable fresh veggies. If I found a flavored variety one liked (eg, carrot) I would shave a few slices of carrot on top of the treat dish, for example. So they're not without merit for finicky fids.