Since you probably just need one-offs without parametrics your best bet is the Google Warehouse. Here's one of the first things that popped up when I searched for "gas station".
To use the parts you will either need SketchUp Pro to convert via 3DS format (my preference) or the ArchiCAD add-on which is Windows only. I believe you can get an 8hr full functioning eval copy of SketchUp Pro which gives plenty of time to convert a bunch of stuff.