All of the above advice is pretty good, especially that Terragenesis website, if you're into the hobby of making terrain. It's very Games Workshop oriented, but there are lots of good ideas there.
Raw materials for making rocks and hills can be found at your local hardware store. Here in the U.S., Home Depot sells pink "foam insulation" board for $5-10 for a huge wall-sized sheet. This styrofoam is much more finely granulated than the beaded kind used to pack stereo equipment in boxes with, and is superior to work with. With a cheap hot wire cutter, you can make all sorts of cool rock formations. Cheap Acrylic paint (Apple Barrel is a good brand at Wal Mart and Michaels) to paint the foam board. Spray paint has propellants that melts the foam, but makes for a cool cratered landscape.
Some companies that make terrain:
Games Workshop makes good trees, but they are very expensive.
Model Railroad shops have Woodland Scenics Tree sets in various scales. Some are premade in a bag (in green, fall and conifer styles). Some come as kits where you put the foam foliage on plastic treebranches and make them yourself (It's pretty easy).
Railroad shops also sell cheap flocked mats which unroll (in theory as a base for your train display). They're about 4'x6' and could work as a grassy field for miniatures to run around on outdoors.
Epicast makes a full line of Sci-Fi, historical and fantasy terrain out of Resin at a reasonable price. The pieces are unpainted. Check out their giant piles of skulls and Easter Island stone heads...
Terraform makes expensive resin terrain too. Aimed at the Games Workshop crowd, it's also unpainted.
"Pegasus" went out of business, i think. They made a whole line of 25mm buildings and terrain (stone statues, fountains, a witches' cottage, etc) that came pre-painted.
Microtactix (I think that's their name) sells cheap PDF files of cardstock terrain and foldup figures for every RPG imaginable. I remember one of the lines was called "village on the cheep...or cheap)".
WOTC has an area on their website with PDFs of full color buildings and castle walls which they posted for free (there are like 12-15 buildings to download) as part of their online support for Chainmail.
Geo-Hex also recently went out of business, but their stuff is still floating around for sale out there. They made a bunch of high quality 4'x6' flocked mats (grass, desert, space) for gaming. They also made an elaborate geomorphic 1'x1' hexagonal gaming terrain system (premade and kits) with which you can design your own (fancy) gaming tables. This is aimed at serious miniature wargamers or battletech enthusiasts.