I've heard a lot about "river cane," which looks a bit like bamboo.  I'm sure there will be websites telling you if it grows near where you are located.  Bamboo may also work.  I have very little personal experience, so I can't help much.  I'd think that any flexible straight thin pole would work, perhaps you could find some saplings that you could cut down and thin if necessary.  Members MammotHunter and CanDo have mentioned atlatls, and I think TechStuff has too, so they will probably have some good advice.

This might help: http://www.internationalatlatlsociety.org/darts.html

This is specifically about river cane: http://www.internationalatlatlsociety.org/cane.html

I didn't have much time to look at them, I just gave them a quick glance.

edit: I have been wanting to do this, if I can find some river cane.  It seems that the darts are harder to find materials for than the atlatl.

http://b16.ezboard.com/bpaleoplanet69529 ( scroll down to Waa link)

you can use ramin, poplar ect for a dart and you can tie smaller ones together to make a longer dart and there are many woods you can make an atlatl out of like oak, willow, ect.... its especially good if it bends

http://www.thudscave.com/npaa/

I made my first darts out of those green plastic-over-aluminum lawn stakes you can buy at any hardware or garden store.   I used duct-tape for fletching, and wound wire solder around the head to bring it to a weight-forward balance.
Non-traditional as hell, but it worked!

I have been working with some river cane that I got from the same guy that sent me my osage bowstave.

This is indeed a "native" bamboo, and normally grows (naturally enough) near water.  It's common all over the Midwest.  You have to harvest it, let it dry, and then straighten it over heat and trim the nodes down.

No reason that regular bamboo garden stakes wouldn't work too...

I plan to make up some full-length river cane darts, with turkey-feather fletching.

As noted above, hardware-store dowels can be joined quite effectively with a length of PVC tubing of the right size.