In State of Decay outposts have supply lockers. Can I place rucksacks of supplies there or do I have to trudge off to a base to do so?
No, you have to go back to your home base to drop them off.
As per the title, in State of Decay where can I find building material resources that can be used to build up my base?
Building materials are all over the place. If you've done some scouting you'll see on your map some potential places, just go where you'd expect them to be in real life. I have mostly found them in construction sites, garages, outhouses or other such places.
Once collected they show up as the pyramid icon in your inventory / HUD
You can also ask the radio operator to put out a call for you to find where such materials have been spotted (although I've not actually tried this yet so not sure what the end result of this would be - a marker on your map, I would assume).
When actually looking for them - they are the larger items that appear when you do a Search of an area. You'll get the option to carry them yourself of to radio to survivors to come and collect them themselves. Don't break them open (which is one of the options) as it'll revert to the constituent parts (pieces of wood, bottles etc) which aren't really what you're looking for. Carry them yourself, you'll get a backpack on your character and will be able to carry them around. Drop them back off at your supply area back home and they will then be available for you to use to expand your base.
You can find it near the farm, where you found Lily's brother, Jacob. Make sure you recon the place before rushing in. Zombies are tricky in the farm and fast.
Sorry, not enough rep to directly comment on your post.
Anyway, calling Lily doesn't show you where resources were recently. The resource actually randomly spawns in a search spot near you.
Basically, If you ran out of that one material in your area, calling Lily to find some will spawn that material near you.