For outside, just snatch images of Google earth (Print screen button and then paste into a word or powerpoint document).
For inside plans google: (building type) floorplan, (building type) layout or (building type) blueprint. So for a good warehouse image I googled [ warehouse layout ] and got lots of good stuff.
If you want to draw your own then you can use GIMP, Incscape or Google Sketchup.
GIMP is a open source raster painting program. In layman terms it's Photoshop, but for free.
Incscape is a open source 2D vector painting program. In layman terms it's Adobe Illustrator or CorelDraw, but for free.
Google Sketchup is a free 3D vector painting program. It's sort of like a simple version of AutoCAD.