If you are sure it is the hamster and not the cage, bedding or sawdust, you can get Johnstons Small Animal Shampoo from the pet shop. Make sure it is not too cold and give it a good gentle rub dry afterwards.
Have to say though, have never ever had to bath a hamster and have had a few over the years.
I had 2 hamsters in the past and never had to wash either of them. Lankeela is right if you do wash your hamster don't let the little one get cold. If hamsters get too cold they go into a kind of hibernation and people have sometimes mistaken it for them dying. Sorry a bit dramatic but give hammy a good drying.
When mine was getting old and less hygienic I used to get a high sided bowl (it can't climb out of) with a little bit of warm water in (only a little so he can run around in it fine) and after cleaning his cage while he played in his ball I'd pop him in and gently rub the water over him so he doesn't get soaked.
He came to associate it with being cleaned out so it wasn't out of the blue for him. He never seemed too bothered by it so may need to reconsider if your hamster gets distressed as they're such delicate little things. He's the only one i've had to give a bath to.
I'd blot him off with kitchen towel then always have a proper towel ready for him to give him a gentle rub dry then pop him back in his cage where he will be distracted by a new food mission and making his new bed.
The running about will help him dry off too and make sure he has a nice cosy bed to dive into and that the cage is somewhere warm and not near an open window or draft.
I'm in the states and i can buy pet wipes. Like baby wipes, but for pets. You use them to wipe over cat or dog fur and dirty paws etc. If you can get those that would be better than giving a hampster a bath as i'm sure she wont take too kindly to it at all.
Mikala