Chainsaws

I have purchased a Husqvarna Rancher 450 50cc 20in blade says Farm Tough on the blade @ Lowe's in Washington NC and service is not what i expected. I am trying to get a replacement chain but I am informed i need to special order this. I do not have the spec's on me but remember it has a 0.325 pitch. So I purchased a chain for a regular 450, pitch is 0.325. Is this compatible with my Farm Tough 20in blade? the man tells me at Lowe's it is not and I need to special Order. Why is Farm Tough not available anywhere else?
Thank you for your time
Inge