Go to Amazon and punch in "garter belt" and you'll find plenty that work.
Generally, they go on over top of your panties/underwear. They will have either two straps per leg or three straps per leg.
There exist underwear that have attachment points for garter straps, but generally the garter belt is a separate piece. As mentioned above, some corsets and girdles have attachment points for garter straps.
If you need to try something on or want in-person assistance, you could try a place like Frederick's in the mall.