The map 'Spire' is more or less the only place to get this. You can find the bugs below the bridge spanning the 2 bases. The best ways to kill them are using the minigun as a turret, the flamethrower, dual shotguns or any other powerful close range weapon you can find.
Choose Small Team Game playlist to make this easier and because you will get the map on that playlist. You should find that most of the action will stay on the bridge and away from you, especially on Capture The Flag.
To keep out of their reach, you may wish to head slightly up one of the two hills heading toward each of the bases. If done correctly, the bugs will hit an invisible wall and be prevented from proceeding toward you, but you will be close enough to still attract them toward you. This may make you more vulnerable to enemy fire because you are not hidden under the bridge. Plus, the bugs will spawn further away, making you need to wait for them to arrive.
You can also find some bugs on the map 'A Heated Rivalry', but only if you actually release them using a computer panel. Even then there are only a few released. Basically, the chances of actually killing enough bugs on that map in time are slim.