I asked this when I first started too. just search for inline fans and buy what looks like a quality one. keep in mind that the fan itself may not make as much noise as the xhaust coming out so a duct muffler may be needed
Dont cheap out.. HTGSupply What are you cooling? just the light? OR u running the inline with a filter an to cool light? Details would help
The blade above is right. More info is helpful to assist you better. Just get the 'Speed Fan Control' and the noise should be fixed. I have a 445 CFM and connected to a Carbon Filter and 600W CoolTube. I use my speed at Low and my temps are 80F under the light. The tempeture of the house is 73F. That should give you an a idea.