I want to make a bamboo flute. Where can I find the bamboo?
in the woods. 
Did you do a search for wood shops in your local area (they may have a bamboo-exclusive section, too) ?
I order my bamboo from someone in California.The stuff that grows here,the internodes are too short.
Try Monty Levenson at…
“Bamboo Sources” are in the left frame, bottom. Then go to “The Japanese Flute Homepage” for plans/instructions at…
http://www.asahi-net.or.jp/~DL1S-YMGC/index-e.htm
…or, look at the “Craft Manuals” section of Shakuhachi.com