I use both the smaller snap clips and larger ones. I use size 60 buttons and I get mine here http://www.craftersvision.com/produc...-12-12-060.htm
shipping is fast and they will refund you the difference if shipping out another method is cheaper. Customer service is really good as well.
I personally open up my snap clips and hot glue them with really hot hot glue on the end....I have had no issues with them as I use them on my own daughter and I live in TX and it is REALLY hot here and I have left them in the car and they do not come undone. HTH!