Some blogs to look at for 3mm terrain:

http://tinytanks3mm.blogspot.ca/

https://nanotanks.wordpress.com/

http://verytinywars.blogspot.ca/?wref=bif

http://2mm-madness.blogspot.ca/search?updated-max=2011-11-23T21:01:00-08:00&max-results=7

Some terrain:

http://www.supremelittleness.co.uk/

https://www.fighting15sshop.co.uk/3mm-middle-eastern-buildings-1023-c.asp

http://www.brigademodels.co.uk/SmallScaleScenery/index.html (Though in 2mm rather than 3mm, and will look smaller than other buildings).

http://bayareayards.virtualscratchbuilder.com/ (my favourites)

http://gcmini.mybigcommerce.com/3mm-city-buildings/

http://www.totalbattleminiatures.com/skirmish/3mm/westerneurope.html

There's also some other stuff - Peter Pig do some 1/600 with their ACW ship range, there's obviously the O8 stuff from Picoarmour, who also do the old Simply 6 range of 1/600 buildings that are excellent too. Magister Militum also does some 3mm stuff, mostly ancients right now: https://www.magistermilitum.com/scale/3mm.html#order=name&limit=36&p=1&dir=ASC&cat

There's some architectural modelling stuff that works too: http://www.microformmodels.com/

There's also some stuff on shapeways, though it tends to be very pricey: https://www.shapeways.com/shops/frenobulax?s=0&section=1%2F700+Buildings+%28European%29

http://toofatlardies.co.uk/blog/?p=281

I've always loved the look that the Lardies got for their Boer War games. I know there is some jiggery pokery with the pictures, but they really convey the openess of the veldt. All pretty simple to set up as well. In one of the blog posts Rich takes you through the various steps invovled.