Its against the law to sell kittens under the age of 8 weeks. I would not trust a shop that has them for sale.

As for adopting an aduilt cat, I think it would be lovely to do so since they are so often overlooked, but make sure its a very tolerant one who will adapt to a new home quickly and tolerate a small kitten. We usually suggest that kittens are adopted out in twos for a reason: they drive adult cats batty.