Noctua fans are reasonably cheap and come in various loudness and flow ratings. Or any two or three pin PC fan with double ball-bearings will probably be ok if it claims to be quiet and high airflow, because you are already paying a premium for the bearings. Get it from any PC parts place, should be less then 15 euros including postage from your country or next door one. Cut the leads 3 cm from the fan and splice on your existing leads (in case they are heat resistant) with heat shrink over the splice. Just use the relevant two leads if it is a 3 pin fan and insulate the remnant of the yellow one.