Fans
pca suggests these:
Farnell 306-2600 40x6mm 5v 5.5cfm (2.6l/s) 26dB(A) ~£9
Farnell 306-2612 40x6mm 5v 4.35cfm (2.07l/s) 22dB(A) low noise ~£9
Farnell 306-2624 40x6mm 5v 3.5cfm (1.66l/s) 16dB(A) super low noise ~£9 (Philip O'Hare reports that the last of those three is quiet)!

Alternatives:
Panasonic panaflo fans available from http://www.dorothybradbury.co.uk/ and http://stores.ebay.co.uk/Panaflo-Fan. They don't seem to do a 40x12mm, 40x10mm or 40x6mm fan at the moment, though.
jaro1.com 40x6mm 5v or 12v
jaro1.com 40x10mm 5v or 12v
www.chenghome.com.tw 12v 40x12mm (cha2512)
www.chenghome.com.tw 5v 40x12mm (cha2505).
RS 197-4558 40x40x12mm 5.3cfm 21dB(A) ~£23+VAT (ELINA HDF-4012L-12LB)
ELINA HDF-4012L-12LB 40x12mm
Sunon KD1204PFB1 40x10mm 8.3cfm (8300rpm) 34.3dB(A)
Sunon KD1204PKB1 9.1cfm (7400rpm) 27dB(A)
Plycon 40x10mm (Sunon KD1204PFB1) 8.3cfm (8300rpm) 34.3dB(A) ~$9
Plycon 40x10mm (Sunon KD1204PKB1) 9.1cfm (7400rpm) 27dB(A) ~$9
ADDA AD0412MX-K90 12vdc 40x6mm 5.2cfm 23.8dB(A)
ADDA 40x10mm 4.7cfm 17dB(A) (cookie_ARM reports this is very quiet)
MOUSER 664-D4006MX-12V (AD0412MX-K90 ADDA) 12vdc 40x6mm 5.2cfm 23.8dB(A) ~$9
Innovative Industrial Co. Ltd (Taiwan) SP401012M 12v 0.06A 6.2cfm 21dB(A)

Choose hypro bearings in preference to ball bearings if possible - they are quieter

Hardware
plastic rivets to hold fan in place: digikey SR-3055B-ND farnell 722-4321or request free samples from Richco US or Richco UK
.

Compiled from various postings including this thread:
http://empegbbs.com/ubbthreads/showflat.php/Cat/0/Number/47765/page//fpart/4/vc/1