Expect to pay $5-10 for a kit (each kit contains two lids). This is a little steep, but since they are no longer made and are so representative of Chessie's railroad practices, I highly recommend you buy any you get your hands on. I have found 4 kits (8 lids) over time and will buy any others I see.
As background information, hopper toppers were temporary lids added to hoppers to make them into covered hoppers. These lids were normally added to handle surges in the grain harvest and lulls in the coal business.
Below is a pic of a real hopper toppers in use.
Dean Heacock Photo
Below are pics of some of my lids ready to go on hoppers.
These kits fit the Bowser 100T hopper perfectly. No need to glue
them down as they fit so well.