Check out WAC and buy the shades you like at the same time.
They have plenty of fixtures which use a variety of shades.

Manufacturers will frequently change the specs of a fixture or discontinue the fixture without notice. It would be much easier on you to buy the replacement shades at the same time as the fixtures.

Thank you, granite man. I suspected that that might be the answer-- it just would have been fun to think that we could look forward to buying additional shades when we came upon ones that we really like.

My kitchen use pendant lighting buy at amazon.

Here is a link that might be useful: pendant lighting

Buy anything you want at the same time as the fixtures.

There are no hard standards governing fixture shade design.

The next manufacturing lot may not be compatible with previous lots.

Some fixture globes have not changed in years, but those are the exception more than the rule.