If you've been diagnosed with cancer, chemotherapy agents (medications) will most likely be used to cure the disease, shrink a tumor, prevent the cancer from spreading, or relieve the symptoms of the disease.  This is done through injections, IVs, pills, or indirect injections to fluid around your spinal cord and brain. Regardless of how the chemotherapy is administered, it works by killing fast growing cells. Unfortunately, weight loss is a side effect of the treatment. Learn how to prevent weight loss, put on weight during treatment, and understand the importance of a healthy weight.
If you're going to use any injectable gear, then of course you're going to need some "darts." You can pick up syringes at your local pharmacy unless your state has certain restrictions. Also, you can purchase needles online. Just do a little searching around and you'll find several places that'll hook you up. Syringes will run you around 50 cents apiece. Note that it'll be more difficult to obtain needles (at least from the larger, more "legit" companies) if you live in California and Illinois. You'll usually need a doctor's prescription in those states. Still, if you look around enough, you can get what you need.