Try calling Whitney Ridge farm in Higganum. They recently moved their place from Durham to Higganum and had put up a tent barn filled with portable stalls while their big barn and indoor were being built. I think they also equipped a few of the smaller shed barns with portable stalls too.
Now the new barn is complete and the horses are in there so I have no idea what they've done with all those portables. They might either have some they aren't using or they may have rented them and can tell you where they got them. Rental places will usually sell extra stalls.
I don't know them personally but their new place is right near my place so I drive by it all the time. The portable stalls they had were really nice.
You jump in the saddle,
Hold onto the bridle!
Jump in the line!