Thrift stores and yard sales are great places to find cheap domino sets. Dominoes last more or less forever, so don't worry about the age of the set.
If you don't have the money to buy dominoes yourself, consider asking friends and relatives if you can borrow a set. Often, someone you know will have an extra set stored away somewhere that they will be glad loan you.
There are also larger domino sets with numbers from 0 to 12 or more, all the way up to 18. The game is played more or less the same way regardless of numbering, but this article assumes you have an ordinary 0 to 6 set.