I recommend the Phobya G Silent 700rpm fan but you will need to use something to get the speed down. It moves a good amount of air with reasonable pressure at 450rpm and is a good alternative. If you run it at full speed, it sounds very similar to the AP181 on it's low setting but I do think it's a little quieter than the AP181 when you slow it down.
If you need a lot of airflow then there is no substitute for the AP181.
I wonder if anyone is using a 140mm fan instead ?