Can't help you with the shopping dilemma as it's been a few years since I've had to shop for a six year-old. But my first question is ... WHY will he not wear snap/button pants? Does he have some difficulty in manipulating these mechanics? Perhaps you need to talk to him and find out why, exactly, he is so reticent to wear pants with this closure. It is possible that he has a lack of strength in his hands and has a difficult time closing snaps or buttons and maybe you might want to consider a bit of "practice" at home.
It may seem a bit cruel but, maybe you need to help him progress from his "little boy" pants into "big boy" pants. If you indulge this issue now, how many others are you going to cave on? Let him know that you cannot find pants for school that do not have either a snap or a button and the two of you need to discuss what to do about an alternative to his prediliction for elastic-only waistbands. After all, how long do you suppose he will be wearing elastic-only waistbanded trousers if he is not helped to cope with it now? When will he move up to "big boy" pants? When he is ten? Twelve? Sixteen? Never?