It used to be that one could get a generic glass lid for a cast iron skillet in a hardware store, but that seems not be the case anymore. When I broke the lid for my generic skillet I shopped around online until I found a glass lid and took a chance on it. It works, but is not an ideal fit.
With name-brand vintage skillets there are vintage iron lids guaranteed to fit, but these are rare and more expensive than the skillets themselves. So when I recently acquired a Griswold No. 6 I expected to do without a lid. Then I stumbled upon a stainless steel lid which seemed to be about the right size, and it was on clearance, so I bought it. It is a Viking 8.7 inch (22 cm).
This lid is a near-perfect fit for my Griswold No. 6, and it is very nice. It is sturdier than my All-Clad lids and has a shallow dome shape. If you have a No. 6 skillet, want a lid, and would be satisfied with stainless steel, consider this one.