where do i find paintings by Winston Churchill? I'm looking to
purchase an original painting or watercolor by Sir Winston Churchill
the late British prime minister.

As these are all collector's items, they're not going to be routinely
available anywhere.  You just have to hope you get lucky, and one
comes up for auction, e.g. at Christie's

http://www.christies.com/home_page/home_page.asp

Or Sotheby's

http://search.sothebys.com/

The following article indicates that prices are likely to be in the
thousands of dollars, up to the many HUNDREDS of thousands of dollars.

http://216.123.50.100/waberigi.htm

If somebody purports to be able to get you a Winston Churchill
original "to order" for a "cheap" price, treat the claim with extreme
scepticism.