There are many places where one can purchase oilskin from. One can purchase oilskin from popular on the web sources such as Amazon and Oilcloth By The Yard.
Vintage oilskin dusters are lovely jackets that can be purchased on ebay. The price of the vintage oilskin duster (depending on the quality) is upwards of $50.
The most common shape for a party hat is a cone.
With steam. Steam the hat then shape it and let it cool Boil a pan of water then let the steam hit the hat until its wet..shape the hat as you wish. Then let it sit for a couple hours.
a sectagon
cone
from a template in the shape of the hat with starched material Butterick sells patterns for them.
If it is a 2 dimentional shape it is called an 'octadecagon'
Wash it by hand; machine washing would destroy the shape of the hat. If its a Top Hat many dry cleaners specialise in cleaning them. There is a plastic hat form that you can buy for the regular brimmed hat. You place the hat in that and then on the top shelf of your dishwasher. Use regular dishwasher soap--not laundry soap. It should keep its shape and color.
Its air inside contracts
A pork pie hat has a circular brim with a flat top. The top of the hat, while flat, has a distinctive crown shape.
Well I think it is very easy get a hat that looks like a Christmas hat but in a circle shape and decorate