Where I live there are about ten shops I could go to - are you rural or city dwelling. If rural, try a local agricultural merchants, or equine supply store. If not, try your council or failing that a company like Rentakill I suppose.
Only problem with going down the poison route is you might not see the proof of the rat's demise - has been said that for every rat you find dead above ground, there can be up to ten others dead that you never find.

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Rat

British by birth
Scottish by the Grace of God

Blog updated Wednesday November 13th