There is no direct bus from Shanghai Pudong airport to Yixing. I think you can take the airport shuttle bus to Wuxi first and then take bus from Wuxi to Yixing. There are many bus from Wuxi to Yixing every day. Or you can take the airport shuttle bus line 5 and get off at Shanghai Railway Station. Walk about 300m to Shanghai General Bus Station to take bus to Yixing.