Did you ever get an answer? We would like to buy additional shelves also.

Drop in at your local home center/big box store like Lowe's, Home Depot, Menards, etc. and ask for shelving. It is "manufactured wood" and comes in lots of different finished, widths and lengths. If you cannot match the exact size of the shelves in the cabinet, you can ask that the store cut them to size for you. They usually charge a little bit for each cut, but it is worth ending up with what you want. Once your pieces are cut to fit, go to the hardware department and ask for plug in shelf supports. Or, take along one that came with the cabinet and match it. Don't bother with the plastic ones, just get the good metal ones. You might even think about replacing the rest of the ones in the cabinet if they are plastic. Those of us who lost a basement to a downsize and now live in small apartments find this service very handy. The store will do this for regular lumber also, for folks who want to hammer or glue something together but don't have the ability to do the sawing. You wold be surprised how easy it is to put together a basic bookcase, for example, once all the pieces are cut to size. Don't be afraid to speak up in the store and tell them what you want done. Paying to have pieces cut and finishing it yourself often times is a lot less in the end than just out right buying something. Plus, there is no shipping to pay. Think outside the box a little bit.