Aquahub.com
Autotopoff.com
Both are sources for floats and there are others.
Waterbug is one water sensor (WB-200). Plenty of threads on that - do a little searching.
specialityalarms.com has a floor water sensor also.
smarthome.com has window, door and humidity sensors.
In all of the above, you're looking for something that has 'dry contacts' (note that Waterbug does but via its controller, not the switches themselves). Dry contact simply means there's no voltage present or required. It works off straight conductivity - open/broken circuit, closed/completed circuit.