This issue of insufficient or not enough milk is one that many women deal with. It is also a problem that is easy to think you have even if you don't actually have it. Just because your baby is fussy and/or cluster feeding (eating frequently) does not mean you have a milk supply problem. There can be many causes including a growth spurt, teething, the baby's personality, or many other things.
If you do, in fact, have the problem of insufficient milk, there ARE ways to increase your supply.
If you need a supplemental nutrition system (SNS), you can get one at amazon.com: http://www.amazon.com/Medela-00901S-Supplemental-Nursing-System/dp/B000NEDGB8/ref=sr_1_1?ie=UTF8&s=baby-products&qid=1293715272&sr=8-1
Here is a video that shows how to insert the tube when the baby is nursing: http://www.youtube.com/watch?v=sHj4xuOPa7k
