This was our dilemma, too. Our sweet boy is 9 months old and wears anywhere from 24 mo.- 3 T depending on the brand (he is around 27 pounds and has always been a whopper even though he was EBF until 6.5 months). Unfortunately, onesies with snaps are few and far between for sweet round little babies. The only place I found a long sleeved onesie in his size was...gasp and with apologies...Wal-Mart. They were on the wall in a section with hangers, I think, and they were only in white and went up to only 24 mo. Target has the rare 24 month onesie, too. Bummer! We also have a Once Upon a Child (used stuff) that sometimes was helpful. Maybe e-bay would be another option. Good luck!
Funny Baby Onesie Boy or Girl 0-3 months to 24 months Embroidered Onesie Of Course I'm Cute Just Look At My Daddy Baby Shower Gift. $12.00, via Etsy.