There are no 120x38mm DC fans that are really quiet. For quiet 120mm fans you either need to get a Nexus (by far the best choice), or a Globe S1202512L-3m and undervolt that. Both of those are only 25mm thick.
The other option is to use an AC fan. I use Sunon 2123XST 230V fans and run them on 120v. A 2 Watt series resistor somewhere in the 270 ohm to 750 ohm range can be used to lower the rpm further to suit your noise tolerance. These are 120x38mm fans and are very quiet when undervolted like this. They have a better airflow vs noise ratio than even the Nexus fans and do better against backpressure. They do have some vibration to them so it's best to mount them with vibration isolators. They are heavy metal frame fans, but only cost about $9 from
http://www.alliedelec.com/Catalog/pf.asp?FN=551.pdf