Linton:
How quickly does the flow detect need to respond and how sensitive does it need to be?
If you have relatively high flow rate and can live with relatively long response time you may be able to make your own sensor that has a pair of thermistors. Each thermistor could operate with different bias and so one has greater self heating than the other. The flowing of the water would cool both sensors and bring readings from the two thermistors closer togther. It may be necessary to place the two thermistors in a bridge circut in order to effectively measure this change and have it be sensitive enough for you.

Hope this helps
Mike Karas