These should do the trick for you.

1. Performance edition - lower decibels than the stock S3 fans: http://www.amazon.co.uk/gp/product/B007RESFYK/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=A3P5ROKL5A1OLE

2. Quiet edition - lower decibels than the stock S3 fans and the the performance edition:
http://www.amazon.co.uk/gp/product/B007RESFYK/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=A3P5ROKL5A1OLE

Both the above are PWM fans, so they will increase speed according to how the S3 feels the need, unless you do the proverbial "blue wire hack" (not sure whether its a blue wire on these fans though). Or, looking at these items' page, you can get the none PWM fans which are a tad cheaper but will run at a constant speed.