You can't really give a hamster a bath, especially a dwarf -- they hate to be wet, and can get colds from being wet. What you can do is buy a small ceramic thing (they sell them at petsmart) and some chinchilla dust (also at petsmart) -- and they will take a dust bath and groom themselves. Hamsters are very clean animals, so you won't need to bathe or brush them as long as you keep their cage clean. But they do like the dust bath. You don't need a brush either, b/c they groom themselves.
If you buy the hamster at Petsmart, I wouldn't bother with buying the transporter cage. They will give you a cardboard carrier to bring your hamster home.
Walmart has LIMITED hamster supplies (at least the ones in NJ where I live). I believe you can get a good cage there (they had a pretty big colorful one at my walmart, so if you can get it there, then that's good -- better yet if it is compatible with the crittertrail or habitrail stuff, then you can expand it as you get money), and also walmart sells Aspen bedding -- which is the ONLY kind of wood bedding you should buy for your hamster -- pine and cedar are not good for them. You can also use carefresh, which I am not sure if walmart has. They might have ceramic bowls too, I'm not sure. I did buy a couple of plastic crocks for my hamsters to eat out of (which are probably good enough), and also some toys. DON'T buy the wheels that they sell at walmart (if your cage doesn't come with one -- it probably will), the ones they sell at Walmart are wire, and hammies can get their feet caught in them -- buy a solid plastic wheel, either a Silent Spinner (which I recommend b/c they won't wake you up at night runnning in their wheel), or the Comfort wheel from Critter Trail.
As a side note, just be careful what food and treats you buy for yoru hamster. Dwarfs are very susceptible to diabetes, so a lot of sugar isn't recommended. A lot of the treats they sell at walmart have honey or sugar. I believe they do have chew sticks and chew blocks which are wood and are good. You can also buy puppy Milk Bones, which are really good treats for them, and will also serve as chew toys (mine love them). They also have hamster croissants which are kind of cute, and don't have a lot of sugar.
If possible, go out and get the bedding, wheel, water bottle, food bowl, and cage BEFORE you get your hamster. That way you can just bring him/her home from petsmart and just put them in their new home. Let your hammy get used to his/her home for a day or two without touching them.
I know this seems like a lot, but don't worry! Winter whites make WONDEFUL loving pets! I've had 2 and I LOVE them, they are very friendly and sweet!