Websites such as eBay and Craigslist often have antique or vintage soda machines for sale. Other companies such as Memory Lane Restoration refurbish and sell various antique items including beverage vending machines.
When in the market for an antique soda machine, another place to look is on a social media site such as Facebook. Facebook has a page that compiles listings of vintage Coca Cola machines and links to where they are sold. One must have an account with Facebook in order to follow a page such as this.
Because of the nature of the antique market, the availability of such items is unpredictable. It is good to look frequently since items can be put up for sale at any time.