How to make international Amazon Affiliate links?

Most of my visitors are from the UK and USA so I would like a way I can post a link which would send people to their appropriate country's Amazon store.

Currently I'm sending people to the amazon.co.uk and getting clicks, but no sales and it may be because the clicks are international.

Is there a way I can do this? If there is an easy and free way to change the link so people are sent to their own country's Amazon store, that would be a bonus!

Thanks in advance.

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