Transportation will be a massive consideration when sourcing materials for a garden. Even sourced locally, materials will move and the transportation of good will most likely use fossil fuels which will emit CO2 into the atmosphere. The less distance a material travels over its lifetime, the less impact it will have on the environment. Exotic materials may look appealing, but the impact they could have on the planet may not be worth the appeal. 
There are some things to consider when thinking about transportation. How was it transported, train, ship, truck or aircraft? Each use a different amount of fuel for the same amount travelled with ships and trains being more environmentally friendly than planes and trucks. (ref) Finding out how a material travelled can be hard, but if the data can be found, it can be useful to know for understanding the impact of transporting that material.
Another thing to considering is the fuel type used. Whilst it was most likely fossil fuels used to transport a good, there is progress to incorporate more sustainable energies. (ref)
To help calculate the impact of imports, the UK government posts data and statistics of the amount of CO2 released depending on the type of transport used. (ref) This, along with the distance a product has travelled can be used to calculate the amount of CO2 released to transport the item to the UK. Some companies do offer ways to help you calculate the environmental impacts too. (ref)