Unfortunately due to recent legislation any developer needs to be able to prove that all materials carted off their site has gone to a licenced tip, so approaching the nearest building site will probably not work.
8m3 doesn't sound a lot but its 20 metric tons, or a full load of a wagon, the sort of quantity you wouldn't want to handle yourself.
By the quantities stated it sounds like you're filling to some depth, whatever material used if placed incorrectly will cause differential settlement between the existing subsoil and the area of fill. Bad filling will lead to failure of the patio over time.
The one thing you don't want to be using is topsoil, obviously this will continue to decompose over time and result in the patio sinking.
Why not get in a local hard landscaper to give you advice / a quote. I know a good one, where are you based? got a couple of quiet months at the mo!