Amazon has Tetra Whisper Air Pump with Minimal Noise and Maximum Air Flow on sale for $4.30. Thank you Kim for posting.
Notes from Amazon:
sale for the prices below after 50% off discount (applied at checkout) when you checkout via Subscribe & Save (Select the later date from the “First delivery on” )
For Use in Aquariums Up to 40-Gallon
Minmal noise with maximum air flow
Has sound damping chambers with a dome shape to flatten sound wave frequencies