1. It doesn't matter what number it is when you disenchant. It will go up as you put in perk points an use enchanting potions. All your doing is learning the enchantment, number means nothing when disenchanting.
2. Black souls gems are the biggest. Those can take a human soul which is most powerful. They can also take a grand soul which is the next step down, that way you don't have to kill humans to fill it. Grand and black have no difference as far as numbers, enchantment does not go up by using a black gem over a grand. Grand gems will work just fine. You can kill a mammoth to get a grand soul.
3. Honestly I've found items to disenchant everywhere but mainly I when I was in a shop I would find something I was looking for in the shop and inventory and I would buy it. The clothes shop The radiant raiment has all kinds of circlets and jewelry to but and disenchant. Remeber you can buy the cheapest one because the percentage does not matter. Buying these items would probably be very expensive for a low level character so it may not be the best idea. I'd wait and eventually you'll find all the enchantments. Just pick up any enchanted weapons, armor, or jewelry when your playing and always go to an enchanting table before you sell anything to disenchant.
Your enchanting numbers start low but putting perk points into enchanting will raise them. I'm maxed out and with using a potion I have some great enchanted item. Example. Ring that adds 67 more health and destruction spells cost 27% less.
Enjoy!