there are problems in buying them, they will look like they are not from the same place, and many times you won’t get what you are looking for(many are not even for games), besides it will look like many other games.
so I recommend you learning how to build them yourself!!
to do so you will need any software that you can model,texture, etc…
also its best to export in fbx
softwares I recommend are , 3ds or blender these will work together with photoshop,gimp and zbrush to create all you can image.
I use , zbrush and photoshop.
autodesk 3dsmax/ are standard in the industry and zbrush has become really popular to bring the models to the next level.
places to learn:
gnomon: ://www.thegnomonworkshop/
digital tutors: ://www.digitaltutors/11/index.php
awesome game art forum:
://www.polycount/forum/