Bamboo is a great kite making material. It grows in poor soils and is competitive with Fiberglass for strength. There are two different types of culms (poles) you can use- whole culms, or split culms.
Whole culms are of limited use for two reasons- generally the ones wider than 1cm are too big for kites, and the ones that are 1cm taper significantly along their length, meaning that they have to be placed only where an asymmetrical spar will work. Generally this means vertically but not horizontally. Whole culms are the strongest for their weight, and some designs like deltas lend themselves to using whole culms. A good source of whole culms is the native North American Bamboo Rivercane, native to the Mississippi river valley.
Split culms from large (5-10cm) culms are more generally useful than whole culms because they are largely symmetrical along their length and can be split down to the needed size. for horizontal spreaders and bows split bamboo is essential. Large bamboo can be found normally by taking an extended walk around your neighborhood. I bet it's growing somewhere
Curing/Drying Bamboo: Cured bamboo can be purchased fairly cheaply, but I recommend finding someone who grows it and cutting theirs. While properly cured bamboo will last for decades and dried bamboo can get brittle in 12-18 months, I think for the purposes of kite building its better to have a lot of free bamboo and replace spars after a year.
Ideally, you'd do cure bamboo after splitting it, because it's easiest to split when fresh. The two most common methods of curing are merely drying and high-heat drying at 120 degrees F (45 C). The latter is called Aburanuki in Japan, and can be done with a fire or a heat gun. Please seek more advice on this technique and don't burn yourself.
Spitting Bamboo Splitting bamboo is fairly easy but tricky at first. Splitting is done by working a knife into the bamboo, and then working it back and forth to force a split. The trick is to always split the bamboo in half, and then in half again. Trying to split off a small piece just doesn't work.
Here's a good video: