Just wanted to post a link to Gems Sensors
Paddlewheel Flow Sensors . I haven't seen it mentioned in this thread, but I have talked about it here before. They are available with pulse train output (Which I bought), Visual indication only, and mabye contact closure at some preset. They wouldn't serve for many scientific purposes, but suited my application.
I can dig up the invoice Monday, if necessary -I believe they charged a nasty handling fee.