I own (2) Fluval Q1 air pumps. They can be found online for $15-$17.
Petsmart online price is $17, though the in-store price is more.
They have 124 liter per hour air output for each of their 2 outlets.
Also, for what it's worth, the brand Danner has a line of "Supreme" airpumps, which are also rebranded as Petsmart Top Fin airpumps that list for less money. You can look at the Danner website to see their specs, and use that info for their Top Fin counterparts.
I have had a Top Fin 3000 airpump w/ dual outlets running a sponge filter on a 20 gallon for a year now with zero issues. It has 2 outputs, so I put a "T" fitting on it to combine the 2 outlets into 1.
Many times Petsmart will have all Top Fin products for 20% - 40% off every few months.