Osh-Kosh still makes snap flies, I believe...http://www.carters.com/oshkoshbgosh/Straight-Jean---Mediu...
GAP has button fly jeans with the elastic inner waistline.
Cherokee (Target) has some snap and some buttonfly.
What I did to help my boys? I let them practice on old pants. Just like the Mickey and Minnie Mouse stuffed toys that helped kids learn to tie shoes, zip up shirts, etc. those work too.
Check on-liine - Zullily, ebay and other on-line stores. Google "size 8 jeans, snap fly" and see what you get.