There are a LOT of costume items available!
Some of them are credit only purchases, but most can be bought with event currency.
All wardrobe unlocks are shared across your region (NA or EU) so you can buy them with alts (Even alts on another shard as long as they are the same region) and it will unlock on all your characters in that region.
Most of the items for sale will just unlock the wardrobe when you buy them, you won’t get an item in your bag, however a few of the wardrobe will be items in your bag that you can sell back for cost price. Be aware that the items you can sell back are also required for cheevos (for example: the pumpkin masks) these will need to be equipped (in your equipment screen not just your wardrobe) to credit the cheevo and then you wont be able to sell them back. You can still buy these items on alts and then send them to your main to equip them for the cheevo, they will stay bound to account after you equip them, so you can send them to all your alts to get all the cheevos on those chars too.
Some of these wardrobes will drop from the minion adventures, mostly the Grim prefix items (you can see a list of special minion loot on the main Autumn Harvest page). There is also a chance to loot the pumpkin heads from the minion adventures or during the Gloamwood instant adventures.