You are referring to muntins, also called grilles or grids. Mullions are the division posts between window units typically comprised of the two frames fastened together and covered with a mull cap.
If you crank open the windows, can you find any stickers or other identifying marks? A Pella expert may be able to chime in with another way to identify them. Depending on how old they are, it may be possible to order the muntins from Pella if you can get the details. If not, you are probably looking at getting some custom-made. If the latter, price them out before committing to that idea, as it could cost a pretty penny.

Try creativemillwork.com for doing custom muntins. Just be mindful that they probably won't match the Andersen profile but then they won't if you get them from Pella.