You may want to consider that these poles will need to be replaced. Many flutemakers come south to Florida in the early spring, to harvest wild crafted bamboo. They might know someone with a stand of the size you are talking about. Many species grow large enough to produce Poles of that length, but you will have to transport it your self.
Heres a company that sells bamboo for construction.
www.calibamboo.com/bamboopoles.html
These are only 10'

People often ask if they can harvest the Bamboo in my front yard to make a fence or something. But only about 8'-12' of each stalk is really usable for that purpose, and very little of it is straight enough.

www.tikifocus.com/Bamboo1.html

Also Nurseries divide roots in early spring, for shipping orders to other parts of the country. Usually most of the cane is removed and It may be too irregular to sell as "lumber." Try contacting some nurseries about this.

www.lewisbamboo.com/cutpoles.html

best of luck
william

Craigs List free has bamboo with some frequency,,,lots of the "donors" want you to dig the whole root out though. 20 feet is longer than whats usually out there but it is around.
Good luck.