Bamboo poles are a great idea and some home improvement stores shoud carry them. The thack can be made out of dry palm leaves, straw, hay, and things with dry color to it.

You can by an actual palm tree at home depot and care for it, and as the leaves die you can make your collection for the roof increase, or just by hay and straw with shouldnt be too hard to find particuarly in the fall (which means you might have to wait a while.

The frame of the tiki hut can be built in three sections using 2x2 cedar. the 4x4 foot thatch will be attached with poly twine. Once the roof is completed, it will be mounted over the 4x4 cedar posts and bamboo pole will be slid in between the thatch and the cedar.

If you live in a subtropical, or tropical area then finding dry palm leaves should not be hard to find especially from mexican fan palms becuase many have a "skirt" of dry leaves attaches to the bottom of its crown.