I need a bloatfly gland to give to a doctor.

Everywhere. But they don't always yield a gland in their loot, so you might have to kill a lot of them before you get one.

I just ignored that quest until I eventually ran across one.

Run west from Sanctuary across the water, guaranteed Bloatflies near the abandoned tank. Literally 1 minute from Sanctuary.

The water resevoir right outside Vault 81 has quite a few floating around, including the black ones which are really annoying.

Make sure that you don't have any other active missions, and then activate that one. You'll get a marker that'll lead you right to one. I was wandering around for a ridiculously long time looking for a bloatfly before I figured that out.

There are a fair few near Fort Hagen too. You will often find them in the wilderness in ruined buildings or dense woods rather than near inhabited camps. Be aware that like other critters, they can be dormant on the sides of trees or buildings rather than always hovering about the place.

Like Quantum Curt said, just activate that mission with your PipBoy and you'll have a marker on the map. It's close to Diamond City.

There's usually a whole swarm of them around the ambulance bay at Kendall Hospital.

Under highway West of ZOutpost 3-4 regular and a black one.