Soda ash is obtained from cooking seaweed on a range or fire. It is an ingredient for making glass in Crafting, along with a bucket of sand. It must then be smelted in a furnace to make molten glass.
Soda ash is usually obtained by gathering seaweed or swamp weed in areas such as Dorgesh-Kaan, the beach near Rellekka and Keldagrim, Entrana, or Karamja.
It can also be purchased at any of the Charter ships for approximately 5 coins. The most efficient place to do this is in Port Khazard due to the nearby bank deposit.
Additionally, after the completion of Swan Song there are nets located northeast of the Piscatoris Fishing Colony which spawns seaweed at a fast rate with a cooking range nearby to turn into soda ash. This is a good method for Ironmen to obtain soda ash at a fast rate.