Please silent, and budget 40 euros / 50 dollars
NoFan CR-95C (huge; covers 1st slot, may touch card in 2nd or cover 2nd slot, too, depending)
Silver stone HE02 (huge; may not fit all cases)
Thermalright Macho (pretty big; may not fit all cases)
Thermalright Macho 120 (smaller, fits most cases)
Scythe Ninja (like the Macho 120, pretty much)
TMK, those are the only ones available, of any reasonable quality, tat can themselves be operated silently. The Macho, with any case exhaust fan behind it,
if it will fit, would probably be the best choice, and those guys stock it.
A Slipstream
PWM would be a better choice, IMO, than the 800 RPM model (which is a great quiet fan, at least for ~3 years), as you could rig it as an exhaust fan, plugged in to the CPU fan header, and get variable RPM, with solid control down to 200-300 RPM, just as if it were a proper stock CPU fan (running it with RPM that low might require help by Speedfan, AI Suite, FanXpert, etc., though, in practice). Then, just get a quiet case and use whatever fans it comes with as intakes. The Fractal Design R4 is available, a good case, and proven quiet (you could probably set up the included fans as 2x intake at 5V, and never have to bother with them again).