I was searching for several different lids and found a link that will be helpful for a great many of the lids.
Pyrex Lids | Pyrex Replacement Lid | Shop World Kitchen
This is where you get the "original equipment" and seems to be cheaper than other sites.
Apparently they don't make it easy to search, or identify the lids to match up with your current set. I thought I would share and make it easier for other people.
(Sadly I have not found a lid for my 10x15x2 dish yet. I am not entirely sure that a lid exists.)