You can get these at Tractor Supply (I copied the picture from their website); I haven't used this particular brand but I'm sure it's as good as any other :). Duck starter is fine.
Hello and congratulations on your new goslings! I haven't had goslings as young as yours (mine were at least a few days old) but they were very wobbly for a few days before they gained stability. What are you feeding them? You could try putting electrolytes in their water as well.