The IRD's are nice, but I've had good results with the Sunrace and Shimano (Tourney) units. The Sunrace come in chrome / silver that are good for keeping a vintage bike looking period correct, but the Shimano's have a 'MegaRange' option that gives you a 34-T 'bail-out' gear if you're expecting some big hills.

Niagra Cycle has a decent selection and most of them are less than $20.

I wasn't sure if a 6-speed would fit on my Bridgestone, but at that price, I didn't have to think twice about grabbing a 5-speed, too, in case the 6 didn't fit.