There are lots of places to get a kitten: from a local RSPCA shelter, ads in the paper, friends and family, or pedigree breeders.
If you're looking for a purebred, try breeding clubs or ask your veterinarian to recommend breeders in your area.
If you've decided on a mixed-breed kitten, a local RSPCA shelter is a good place to start. Don't be surprised if they ask you lots of questions and want to carry out a check of your home. All good rescue centers will want to make sure they're putting their kittens into safe hands.