H.W.
I'm partial to the Hanna Andersson training unders; they are the thickest, most absorbent training pants I've found. No snaps, just pull up or down to get on/off. Of course, they aren't waterproof, but the purpose of helping a child with using the toilet is to help them understand the body's cues, and being wet is one aspect of this. There are no pockets, etc, they cost a bit more (about $28 or so for a 3 pack) but they wear well and as I said, are very absorbent for those little leaks and drips.
I'd also suggest googling "Toilet Learning" online--University of Michigan has a great site. Kids often begin to show some interest in the toilet at this age, and you might want to see which other signs of readiness she's showing, just so you don't get too far ahead of the game (or become disappointed in her or yourself). In my years working with children, this is probably the best resource I've found. Written for child care providers, and definitely one of the strongest, most supportive approaches I've ever seen.
Not that you necessarily need this site, but it's worth checking out. It's saved me a lot of sanity!