Hairs, skins, and eyes can be either your base avatar or an item.
You can change your base avatar by purchasing hair and eyes from Salon Durem (whose options never run out or disappear) or skins from Skin Tyte (ditto).
Over your base avatar you can lay items that change your mouth, skin, hair, arms, legs, etc. For example, I am wearing a skin from
Padmavati's Lotus and hair from
SDPlus #163 Jaws.
When I go into dress up and take off all my items, I look like this:
By special items I assume you mean non-shop items?
There are a few ways you can get those.
~ Event items are given away by the site for free during special events, we just finished up an Easter event that awarded items for clicking on eggs that NPCs posted. Each event has its own rules for how to get items.
~ Sponsor items are items that other companies pay Gaia to make and are usually given out after watching a short advertisement for the product or answering a quiz or poll. These items are usually found in the announcements so keep a look out for them!
~ Daily Chance grant items: Some items are unique to the daily chance cart at the top of each landing page. There are also unique daily grants you can get from aquariums (if you have fish in your tank) and zOMG! (if you know which NPCs to talk to, zOMG! is a bit complicated.)
~ Exchange items are items
sort of found in the shops. Some shops like Ruby's Rack, The Ol' Fishin' Hole and H.R. Wesley have a button in them to take you to a place where you can exchange game items for avatar items. Trash picked up in towns can become paper hats; fish from the fishing game can become a large assortment of hats, eyes, etc; and credits from the word bump game can be traded in for caps and gowns to name a few.
~ Grant link items come from lots of places, they can be slightly secret so keep your ears and eyes open for people talking about them and sharing a link.
Here's one to get you started.
~ Cash shop items are items that you just straight up pay gaia money to have. The cash shop (La Victoire) is always changing stock so items that are no longer in the cash shop will cost more gold on the marketplace because they are harder to get.
~ RIG items are basically cash shop items but they can only be found by buying a certain item that gives a random grant after playing a game of chance. It's hard to describe if you've never used one. It's a bit like opening a box that could contain ugly items or super awesome items but you're never sure which.
You can, of course, buy ALL of these on the marketplace if other users are willing to part with them. Some can cost quite a bit of gold so find a way to make it that you enjoy. :3