Could you get your 6 and your 9 year old helping out with cooking and cleaning or with entertaining the littler ones?
Perhaps you need to get in loads of art stuff, DVDs and books so that there's plenty to entertain the older ones while you're taking the time to get your supply built up. It's great that the baby forces you to sit down for a bit while you're feeding because otherwise it can be hard to stop - ever!
Having a lie down in the daytime really helps (though I know how hard that is with a two year old). I find sleeping with the baby works well because you can build up your supply at night with minimum effort for feeding.
Take any offers of help you can get for the next few weeks.
By the way, FIVE? Well done!
Congratulations and good luck.