The workers at fish stores can rarely be trusted to give good advice.
Could you tell us more about the symptoms of the dead fish? Tell us your tank water parameters, tank dimensions, what filter do you have, etc. We can help you figure out what really killed your fish.
The only real reason to get an air pump is if your water doesn't have enough oxygen in it which would make sense if your filter is internal and is not causing any surface agitation. Surface agitation is what oxygenates the water and this is what an air pump does.
If your fish died of suffocation (lack of oxygen) then it would be gasping for air either at the top or the bottom of your tank. The rest of the fish should be doing it as well. Are they doing this?
When you say that your filter has a spot to attach an air hose, is it to power the filter? Some filters are powered by air pumps. Does it have a built in airstone? If we could get a picture and/or the name of the filter we could better help you.
As for air pumps I recommend
Aquarium Co Op's USB Nano air pump.
It is almost dead silent and a good choice all around. Just look at the reviews.