How do people buy clothes for their avatar? I have seen some really cool wings and horns but idk where to get them

Avatar items are obtained in many ways
> non-sylesti item drops (depends on enemy. Most of the ones you see people wearing will be drops from festival zone bosses)
> avatar shop
> from other players (via trade broker, sales forum, fest activity (ToT in fallfest's case), general gift)
> avatar item boxes (eg, [Avatar Item (Fall - Retired)] from fest tasks)
> haunted house (or seasonally applicable fest equivalent)
> rare drop from helping other player's brews (or seasonally applicable fest equivalent)

might have missed something

Hi, currently there is a festival going on where you can fight bosses that have a chance at dropping avatar items, located here:
https://www.sylestia.com/explore/newzone/?zone=umbrasveil#explorewindow
There's also the fest shop, which under the avatar item section, has retired avatar item boxes that have items that aren't currently obtainable from this fall fest, but were available in previous fall fests, for sale for the fest's currency:
https://www.sylestia.com/shops/holidayshop/#shop
Some players may also be selling these retired boxes from fall fest, and other seasonal fests, under the miscellaneous section of the trade broker.

Other avatar items can also be dropped from main gameplay(Ones garnered from completing parts of the game, or dropped from defeated enemies), and in the Lost Grove(The current endgame), you can obtain avatar item patterns from defeating enemies, which can then in turn be crafted in the barracks of the Lost Grove(Which has to be built first).
For avatar items that you don't currently have access to, as well as items currently unobtainable to get on your own(Other fests that aren't currently going or calendar items), you can check the Trade Broker(Link), or the Item Marketplace Forum(Link) to see what other users are selling.

There are also avatar items available with the site's premium currency diamonds and peerless avatar tokens, here:
https://www.sylestia.com/shops/avatar/

I hope this helps explain a bit!
There are a few ways I didn't include(Slipped my mind), but Scathreoite included the ways I missed in their post, so thank you to them. ^^

Though, as a newer player, as a welcome to the site, I went ahead and sent you some spare avi items I have.
To use them, you'll first have to accept them from the Player Trades under the Trades Tab. After that, they'll be added to your inventory. Click each item individually/one at a time, then click the Use Item button. When an item is selected, it'll be lit up blue.
After that, the items will have been added to your wardrobe.

Thank you so much for your answer! I will try to have a go in the festival zone and see if I get lucky!

Omg thank you firstly for such a comprehensive response, but an even bigger thank you for the clothes! I really appreciate it and am so excited now to go see and try them! Truly thank you! :D