In such cases we ask colleagues from experimental stations run by the state or universities to allow us to dig on their grounds. One extra advantage is that you often get access to a number of extra information of site and soil.
I think after official permission, it is better to make pit and describe the profile to decide the soil series and its Family level classification. And then you can select the depth of sub-soil from which you want soil collection.
I think it is better to contact such state research Department(s), where their researchers already have complete information of soil profile and information about site studied in some other projects they run or completed. It will save your effort.
Very interesting , I fully endorse the comments of Dr Ghafoor. But Inaki , why are you willing to buy sub-soil instead of surface soil . Can you please elaborate a bit about your preference for sub-soil purchase ?. What kind of experiment , you are looking at ? please Inaki