I use Johnson and Johnson no more tears baby shampoo. You can buy it in the baby section at Wal-Mart. It is inexpensive and the Wal-Mart brand of this shampoo works just as well but does not smell as good, in my opinion. I have never had a problem with a cat having an allergy to this (I believe that most baby shampoos are hypoallergenic anyway.)
Another shampoo I have used is Aveeno Baby shampoo. This would be good for a cat with dry skin. The Aveeno I used has oatmeal in the formula. So if you had a baby with sensitive/dry skin this might be the better one to use. It is quite a bit more expensive than the Johnson and Johnson brand.
I was mine three every month and never have had an issue with using these shampoos on them.
Oh, too, you might want to invest in some hairball remedy paste from the store if you plan on bathing regularly. If I do not give my cats some after a bath, it is hairball city for like the couple of days. Since they do more grooming after a bath, they ingest more hair than normal and hairballs become more of an issue after bathtime, at least in my household.