One of my friends sent me this e-mail

The main thing with doing heavyweight dying is that you sometimes have to adjust the strength of your mordant, and do other pretreating to make the fibres ready to accept dye. These things can get to be expensive sometimes, but its more important, from what I understand, to spend a bit more money on the chemical stuff, because once the fabric is ready to be dyed, you can use a lower grade dye, and the colour will be okay. Type of dye you want to use also depends on what colour you are trying to achieve. Higher grade things can be finicky, and I know with paints, certain brands have certain colours that are really strong and good, and others that aren't. Darker colours, especially, are hardest to get, and for a heavy weight fabric especially, can require quite a bit of chemical treating.
Her other point was just to drop into your local crafts shop and ask. If they don't know specifically, they might have a good reference for you.