One of the easiest ways to buy the correct spare parts for a coffee maker is to use the brand and model number to search on eBay. The fact that the auction site is used by both businesses and private sellers means that most common parts are readily available.
Alternatively, specialist websites such as Coffeemaker-SpareParts.com may be able to help. This site is based in Europe, but ships around the world, and it is especially good for sourcing parts for more obscure imported machines. As a bonus, the site also provides factsheets and troubleshooting instructions for a range of repairs