Where you live becomes an issue here. Indigofera tinctoria is only possible with fairly long summers and hot temperatures. Otherwise, you won't produce much dye.
www.superseeds.com sells the seeds (I have ordered from them before). An alternative for more northern growers is polygonum tinctoria, but the seeds are not easy to locate. Gram for gram, these two species have roughly the same amount of dye in the leaves. A third usable plant is called woad and is in the cabbage family. The dye content of the leaves is significantly less (1/6 to 1/10
th) than the above species with the bonus of it being hard to kill and easy to grow. In some localities it is a noxious weed. These seeds are available through that website as well. You can also synthesize the dye yourself (though not in great yields (10-25% of theoretical I think), but avoiding high temperatures) by dissolving o-nitrobenzaldehyde into acetone and adding (slowly! The reaction should be quite exothermic) concentrated NaOH solution. Methods with higher yields require conditions much more suited to a well equipped lab.