Jackie, to the best of my knowledge nobody currently reproduces that filler panel. Is yours damaged or rusted beyond repair, because they are fairly easy to patch by welding in new metal. Post a picture and I'm sure we can probably help your out. Also, post in the 'parts wanted' section and someone is probably parting out a Dart and may be able to help with a used one. Good luck.
I'll get some pics if I can figure out how to post them. It's not terrible but it has rust under the glass recess, on the surface where the vynil top was and in the drain trough next to the trunk seal. Any one of the areas alone wouldn't be too bad. All three will make for lots of little patches. Compound angles and curves too. Back glass has to come out so a whole new one would be easier. Car is a little wierd. Inside the doors look like an '06 as well as some other areas. Others look like a fishing junker?? Thankfully most of the damage is confined to the fill panel area.
If it's rusted under the glass recess I'd try to find a good used one, preferably from a donor that never had a vinyl top. Buit if you're stuck, that one can be repaired if you're half good with a MIG.