Sonotube

I bought mine from a concrete supplier, in a 16 foot length. They wouldn't offer to sell it in smaller lengths, so the narrower your tube, the less you can probably expect to spend. I paid around $70 USD for a 16 inch by 16 foot tube.

(About 40 bucks total for my finished enclosure, and I still have enough material left to build two more subs... not to shabby if you ask me!)
 
I went to a concrete supply company and purchased 20" diameter sonotube. I went in with measurements and they cut it to length for no extra charge (27"). I found that concrete suppliers offer a much wider variety of tube diameters than general hardware stores.

Good luck on your sonotube project!