You can buy split-flap clocks at MoMA [0] for $650. Not cheap.
If you want to see a big one in person, the TWA Hotel at JFK Airport in New York has the original split-flap departures display made by Solari di Udine [1] (the same company makes the MoMA clock). Solari also makes other split-flap devices [2].
Not to get off on a tangent, but the TWA Hotel is a very cool place to visit [3].
Split-flap clocks don’t have to be as expensive as the Solari clock. They don’t seem to have the economies of scale to be super-cheap these days, but their mechanisms are fairly simple (essentially analog clocks with really weird-shaped hands) and can be had in the ballpark of $50-$100 if you’re not buying what is essentially a name-brand art piece.
Alphanumeric split-flap displays are way more complex and expensive; you won’t find those anywhere for cheap. The higher-end Solari clocks (the ones with added complications like day and date) likely sit somewhere in the middle; I wouldn’t be surprised if these resemble big watch mechanisms with similar complications.
There's also the Vestaboard (about 3k) if money is no object. Compared to the price of building one from scratch it's probably not a bad deal. 132 characters including colours. It is essentially an art piece, but it does look amazing.
Alphanumeric split-flap displays are way more complex and expensive; you won’t find those anywhere for cheap. The higher-end Solari clocks (the ones with added complications like day and date) likely sit somewhere in the middle; I wouldn’t be surprised if these resemble big watch mechanisms with similar complications.