Choose a color that matches your garment. When you go to the fabric store to purchase your bias tape, take along a scrap of fabric from your garment so you can match the color. The display of bias tape is often found very close to the thread and zippers. Most people like to select a package of bias tape that is close to the color of the garment, although it doesn't really matter that much because it won't ever be seen by anybody when the costume is being worn. As shown in the picture, it has a texture of bars across it.
Some places sell it in cellophane packages containing one-yard pieces.
Others have a large roll of it, and you can have the store cut the amount you want. Grosgrain Ribbon As noted above, if your garment is quite heavy, it's better to use a drawstring made of grosgrain ribbon instead of elastic. The section of the fabric store that sells ribbons and trims is usually near the Notions section. Grosgrain ribbon is a sturdy ribbon that has a ribbed effect. It comes in a variety of colors, so you can probably find a color that comes close to the color of your garment.