Modifying an off-the-rack garter belt is one of the easiest costume projects to tackle, so there's really no excuse (insert angry cast director or costume mistress rant). Our cast's minimum standard only asked that floorshow corsets and garters be solid black, but it looks so much better when the performers go the extra mile with their garter straps and sequin gauntlets. A spot-on kickline is killer.
The shape of the garter belt is a traditional 4-strap style with two side straps added. And since all the straps are covered with red crinkly stuff it doesn't even matter if the added elastic matches. You can order extra garter clips on eBay for cheap. Make sure you secure your strap lengths before applying the red crinkly ribbon (wired Christmas ribbon works great and it's easy to find).