Temple garments can be bought at a store owned by the LDS Church called Beehive Clothing, which is located at most temples. In Utah, where there are a lot of members, Beehive Clothing stores are located in some shopping centers for convenience. You can also order temple garments online through Church Distribution. Garments are worn after one has gone through the temple and made the covenants that the garments are a reminder of. For young men and women who serve missions for the Church, they go through the temple prior to leaving on that mission. Otherwiswe, it is normally expected that you would go when you are ready and prepared to enter the temple. If you joined the Church as an adult, then one year after your baptism you can go. If you grew up in the Church, but did not go on a mission, then the Church usually expects you to wait until you are about 21 or are planning on getting married in the temple, whichever comes first. There are exceptions to that age, mind you, as my wife went through the temple and began wearing garments at an earlier age than 21. 21 is not so much a rule as it is a guideline. Hope that helps answer your question.

To clarify, a person is "ready" to go through the temple and receive their covenants when they are ready to make and keep serious covenants with God. It is a highly personal decision that is reached after prayer, contemplation and interviewing with a Bishop and Stake President. It is a major life step, and one that shouldn't be taken lightly.