The pouch can be purchased through the Rewards tab on the Temple Trekking/Burgh de Rott Ramble noticeboard. Choose the 594 box and click the Claim button to make the purchase. 50,000 coins buy one Ghast familiar pouch and 25 Ghastly request scrolls. Summoning the familiar in Morytania is a requirement for completing the elite Morytania Tasks, and thus also for the Completionist cape.