Donald.
I felt sure there would be people here far better qualified to give you an answer but, If your experimental horse artillery look like these here:
https://weaponsandwarfare.com/2015/09/19/prussian-horse-artillery/Then I would agree with Paul that the Revell 7YW Artillery set might be the best source for figures. Or, perhaps, the Imex AWI artillery sets. The guns in these sets are only 2 or 3lbers though.
Wikipedia says Fred's experimental horse artillery used 6lber guns. The Zvezda Great Northern wars artillery sets have nice 6lbers. The Strelets sets from the same period have guns that might be 6lbers but not as nice. HaT's late Napoleonic Prussian artillery has a 6lber barrel, it looks like it's swimming in the carraige when you mount it but you could perhaps do some carpentry on the carraige. The gun in HaT's 1806 artty set is apparently a 12lber.
All of the limbers in these sets look pretty much like the one in Pauls link so I'd guess that's what you're looking for, it's pretty much the same as HaT's Napoleonic French limbers too.
How many figures do you think you'll need?
Edit: Thomas posted while I was typing this, those Hagen figures look very nice too.