Balista's are still sold by guilds, but they have far less restrictions than cannons and more importantly, easier to obtain on the black market. They're worth about 500 GP from reputable sources, but some places might sell higher or lower depending on the rarity, reliability, or if it's hot (aka stolen) or not.

A ballista has a range of 120/480 feet, and can do 3d10 Piercing Damage. It does [i]not[/i] do extra damage to structures however, but can still ignore a structure's damage threshold. It takes one action to load the weapon, one action to aim it, and one action to fire it. It cost 1 GP for your standard Ballista bolt.