Normal backpacks are sold by NPC vendors, for about 5 gold each. There are many larger or weight reducing bags available as loot from certain monsters, as well as made by players and sold in the bazaar.
A relatively easy one to make yourself with 10 slots and some weight reduction is the Reinforced Medicine Bag. It only requires a Tailoring skill of 68 to be trivial and materials-wise just six Spiderling Silk (sub-combined into three Silk Threads and then those into Silk Cord) and a Thick Grizzly Bear Skin, which are both easily farmed at low levels (or just purchase the bag in EC Tunnel for around 20-25 plat).
Mixing bowls hold 10 and are 1 silver. You can use traders satchels for your bank items. They are cheap (you can get one free from tutorial) but don't use them on yourself as they have no weight reduction so everything will be real heavy. I really like this site for a nice list starting at level 1 http://www.paullynch.org/eqguide/guides/bags/
