You can find these garments on Ebay auctions under the name Mormon Temple Garments. You can also contact a member of the church to direct you to a distribution center.
This is more popular to find in heavily populated Mormon areas. If you do not live in such an area, you can always search and order online. Some garments are also handmade.
LDS garements or Latter-Day Saints garments can be purchased at a variety of stores. One convenient site on the Internet is the Distribution Center for Latter-Day Saints.