At first, I thought the answer was simply that you can only use だそうです (because that's all I have ever seen), but today I read the entry for そう in the Handbook of Japanese Grammar (1940s) by Henderson (PDF via arxiv.org) and was surprised to see that the author listed both きれいなそうです and きれいだそうです as ways of saying "They say (she) is beautiful." He went on to say that it would be unusual to hear such a soft sentence as "きれいなそうな" in Tokyo, at least used by men. He didn't elaborate much, however. This same book said that the word is often written as さう (I believe this is obsolete today) and mentioned that it is an abbreviation of さよう.
Is "きれいなそうです" correct, or perhaps an outdated usage of the form, or is the author mistaken? Could anyone shed some light on this?