You might think that all wood you buy nowadays will be responsibly sourced and ethical. This isn’t always the case and wood can still be brought in from abroad, where it might not have come from sustainable sources. To find out whether your wood is sustainable, all it takes is a little research on your part. The main thing to look for is a quality timber supplier, whose reputation rides on their sustainability. You should speak to them about their sustainability policies if you have any concerns at all, but they should have information like this available on their website.
You should also look out for an FSC logo when buying your wood. This indicates that the wood comes from a sustain ably managed forest, where trees are replanted once they have been cut down. The trees are looked after by the Forest Stewardship Council, so you know exactly where your timber has come from.