Someone already posted this long time ago, so I may be repeating, but yesterday I bought 2T size undies for my 11month old, and they fit perfectly, much better than Gerber's which are too high. DS is rather small for his age, still wearing 6-9 month clothes, so I think they would be good for younger babies too. And they were on clearance for 1.99. Normally they are 3 for $12. The only difference between the regular and clearanced ones was the fabric pattern.
They are not exactly training pants, I think they are meant to be a normal underwear as there is no absorbent layer, but I don't care about that. They are soooo cute on DS's little butt!