There are 2 easy ways to get Molten Glass in Runescape
1: Make it yourself.
> Required Items: Seaweed, Bucket of Sand (Sandpit + Bucket).
> How: Cook the Seaweed on a Range to create Soda Ask, Use Bucket of Sand (or Soda Ash) on a Furnace.
> Entrana has everything you need to create (and craft) Molten Glass (Seaweed, Sandpit, Buckets, Range, Furnace, Blowing Pipe)
> Find another player with some, either in person of using the Grand Exchange, and buy it from them.