There are many types of instruments that can measure liquids flow rate. Orifice, turbine, PD, sonic, annubar, vortex, and many many types. To properly select a type, you shall define at least the following:
1- liquid name (cleanness, corrosive…)
2- Flow rate range, Pressure, Temp. viscosity, SG
3- Line size
4- Accuracy (fiscal measurement ?)
5- Output form (4-20 ma, pulse, local indication.
If you are looking for industrial manufacturers then you can check
Emerson (Daniel Ind.), Brooks, FMC,
There are other manufacturer who are specialized in low flow measurements