Some you can buy at the RC Guild when you have level 50 RC, or you can kill abyssal monsters and they drop them as random drops.
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you can get an easy one at Draynor Runecrafting Guild for a small price.
It's just to the west of varrock.
Your 1st small pouch, can be obtained from Mage of Zamorak when you do a mini quest for him to unlock the abyssal dungeon used for quick runecrafting, a small pouch is also your reward, if you lose that one... --------------------------------------------------------------------------------------------------------------- ...Option 2, kill Abyssals for pouches, they tend to drop small, medium, large and giant pouches, bare in mind they all req. diff RC levels to use.
about 70-100 days when you have got the level for nature runes
It's hard to tell exactly how much you will make from Runecrafting, all we can say is you should make a lot depending on what runes you make.