I built my own immersion out of 3/8" copper and it works great. Whole thing cost $30. The two most expensive parts where the copper tubing and the fitting for my faucet. But I'm the type of guy who would much rather build something than buy it.
LHBS sells immersion chillers for $60 but I believe those are 1/2".
Primary: empty :(
Kegged: empty :(