8 slot bags can be made by crafters (tailor, leatherworker, armorsmith) with 20 tier 1 materials (jute scrap, rawhide scraps, copper ore).
4 slot bags can be purchased from vendors for 32 copper.
You can have 250 of each crafting item stored.
Crafting Material / Cooking Material / Mini-Pet Storage.
Here are some things I've found out about GW 2 to help me plan my storage space. The main problem is that I love to craft and quest and always have a ton of those items on hand. I always run out of inventory space in any MMO I play.
Guildwars 2 Inventory And You! - A Guide To Maximizing Inventory In GW2