You most likely want to use the closest cubemap entity for reflections, but you can also use handmade cubemaps from tf2_textures_dir.vpk\materials\cubemaps or make your own. However, making your own cubemaps is not an easy process.
Use the closest cubemap entity
$envmap [texture filepath]
Use a handmade cubemap texture.
$envmapmask [texture filepath]
Use a black and white image to tell the material which parts are reflective and which ones aren't.
White = Fully Reflective
Black = No Reflection
The links provided show you more parameters as well as explaining the basic process of making a cubemap VTF.