If you saved the fill strip inside the track, reinstall it. I've never seen it offered as an aftermarket item, I think the strips are installed at the factory.
Another way of doing this is to place a bead of sealant inside the track, tool it to a small dimension (the dimension of the original fill strip), and let it dry to harden. Then, when you install the glass, place more sealant in the track to seal the glass to the track.
You may have to make your own apparatus for tooling the silicone if you're following the above method. I've used a tuck pointing trowel for this type of operation, they come in very narrow blade widths. Tape a stick or piece of metal to the blade of the trowel, set to the depth that allows for the dimension of the fill. Drag it through the fresh silicone and wipe blade occasionally. The trick is to put enough sealant in the track and not so much that you fill the sides of the track. You don't want too much silicone in the track so that the glass won't fit.
