I bought a diaper bag from Mimi maternity before I had my first baby. I actually ended up using bags that were not diaper bags at all. I personally never liked the look of the bags made for this purpose.
Coach has many bags of the right size for carrying all you need. I got a tote from there and another from Ann Taylor and pretty much just use use those two. My advice is to find any big bag that you like and use it as a diaper bag. A good friend of mine got an awesome one from Dooney and Bourke that she uses all of the time, it just depends what you like.