Hugo Lloris and Rene Adler are the best after Casillas - but in the first season you can only get them for around 30 and 18 million GBP respectively.
Maarten Stekelenburg, Joe Hart, and Diego Benaglio are good goalkeepers and are cheaper than Lloris and Adler, with the exception of Hart who goes for over 20 million in the first season. However, City usually buy a better goalkeeper in coming seasons which means Hart is available for much cheaper.
Honourable mention to Igor Akinfeev and Sergio Romero. The former is probably the better keeper but both haven't really been as good as those above in my experience.
Guillermo Ochoa is a bargain at 5 million pounds and performs better than his stats would have you believe. Carlos Kameni is also a bargain and is available for a similar price.
Alphonse Areola is a good buy for the future with a [POTENTIAL ABILITY SPOILER] fixed PA of over 190 I belive, but his CA is at 120 odd at the start which means he takes a while to develop and is erratic at the start of the game.