There are no part numbers for lid cardboard. You just need to call Festool service and ask for it, should be free. The one I got for the RO150 was free but I also ordered a Sys 3 T-Loc at the same time. At a minimum, you'll have to pay shipping, maybe a buck or two for the cardboard, worse case.
You'll also need a set of the 4 white "plugs" that hold the cardboard to the T-Loc lid, these are a paid part.
You'll probably want to get the piece of foam that glues to the lid and helps hold the tool in place, as the old piece may not move over properly (depending on where in the lid contour it needs to fit). This is a no-part item like the lid cardboard.
And you can get the base insert if you don't want to do the small modification necessary to re-use the existing one. They are usually $9.95