Foal slips don't last long simply because they grow out of them so quickly; IME you're far better off with an adjustable (nose and throat) foal headcollar. Usually plenty on ebay; Jeffries are nice as are any by decent saddlers, another fair brand is Ascot, more bargain range but well made and sewn. Most of mine, all old English leather, are years old and still as good as new except where they've been chewed by other cheeky monkeys!
PS: don't, if you can help it get anything other than those with a central fixed ring at the back of the nose, those ones that slide from side to side are not good for foals as they're not a definite enough action; I won't even buy those for older horses either, a cheapskate way of finishing them off.