As for 6mm stuff……Irregular Miniatures has a mortar in its 6mm 18th Century range, but 6mm is roughly 1/300 not 1/700. Peter Pig has mortar rafts in its 1/600 ACW naval range, but IIRC the models are one-piece so you might have to cut the mortar free of the raft to use them.
On the other hand and probably more useful for you, albeit perhaps a bit more complicated…..
Bay Area Yards had 1/600 ACW mortar rafts with the mortars as separate castings, and maybe had mortar castings for land batteries. Bay Area Yards shut down its web store a few years ago, BUT
I believe the owner of the store is a member here and he occasionally posts notices here of castings he has for sale on eBay-I've bought from his eBay store once or twice.
Also, there's a Facebook page for Bay Area Yards (with a photo of the mortar raft) with some indication that he accepts product orders by messages sent to him through Facebook; the most recent reference I saw there to an order was January 2024, so maybe you could order some 1/600 mortars or mortar rafts from him through his Facebook page-
The name of the Facebook page is Bay Area Yards, so it should be easy to find.