A number of sources are now offering 3D printing models (STL files) via DriveThruRPG or Wargame Vault.
Fat Dragon Games, a company that started with print-and-play paper models, has the Dragonlock line, which includes both monster figures ($5 for five figures) and dungeon terrain ($10-20 per set). The dungeon terrain sets include stone walls, pillars, stairways, natural caverns, and also hazards, traps, and accouterments such as a treasure chest and an altar. Dungeon levels are stackable and sections can be held together with printable clips. Fat Dragon Games, by the way, is also running a Kickstarter for Dragonlock models of above-ground buildings.
Dicey Ventures focuses on Chibi-inspired architectural terrain features in 28 mm scale. Prices range from $8 for the model of a small bubble-like Water Dwelling to $32 for a Fortification Bundle that includes tower, walls, and gate. My favorite is the Arcane Library ($15).
French minis manufacturer Via Ludibunda sells three different building sets: Basic Houses ($28), Specialty Houses ($28), and Drow Architecture ($30). The Basic Houses set includes models for a variety of components (floors, roofs, pillars) that can be mixed and matched to form custom buildings.
Dozens of different monster figure models are available from mz4250 and all for free. Among the more impressive are a Manticore and a Hippogriff. There are also collections for Lycanthropes and Dragons.
Carnivorous Plants ($4) and Leeches ($6) are available from BSG Miniatures.
Rocket Pig Games offers a mix of monster figures, terrain features, and props. Example monsters include a Troglodyte and a Rust Eater ($1.50 for either). In the category of terrain, the company sells models for Marble Columns ($2) and Traps ($3.50). Among the prop models are Cage (50¢), Sundial ($1.50), and Cauldron ($1.50). But the best have to be the ones that combine prop with monster—the Mimics!
Axolite Gaming’s specialty is a system of interlocking tiles, walls, and doors. The company sells science fiction style and fantasy/dungeon style sets ($10 per basic set, $18 per expansion set). Also mini expansions for an inn and bar ($6 each).