I used to work for a landscape gardening company before starting my studies. We used to get most of our materials from HG Commercial (
http://www.hgc.ch/). You can pick up things there without being a company too. Minimum order is usualy 1/4 m^3 for concrete, soils, sands etc. Last year when I needed a few tons of gravel I found a local building company who had a truck and asked if they could deliver. Was no problem.
If you want filling materials (where no plants ever go), then the cheapest stuff you get is usually "Reco"-Kies, which is "recovered" gravel, so shreddered concrete mixed with sand and gravel. If you want to plant stuff in it, you want "Humus", not sure all the HGC-Places store it and the quality can very enormously, depending on the soil you want and where they get it from. Our company sourced it from a few local smaller suppliers, depending on the needs. I'd suggest if you know exactly what you want, hire a truck, drive to HG Commercial, pick it up. Or alternatively find a building company that can deliver it to you - should be lots around, just see who's building in your neighbourhood. The delivery will probably cost twice as much as your goods, but such is life... They are usually quite frienldy guys too and happy to give you advice on what to get and where to get it too. Good luck!
....oh and if you calculate how much you need, round DOWN to the neares 1/4 cubic meter. I needed about 750l last year and becasue they just grab it with a massive shovel and throw it on the truck, I got about 1500l - peanuts for them, but a lot for you if you have to wheelbarrow it up your driveway...