Hi,
OBJ - is a text file containing object geometry and properties
MTL - is a text file containing object materials
TGA - is a texture image, like jpg or png
Basically MTL file links your textures (as TGA) to you model (OBJ)
When you buy OBJ model without MTL file, seller probably assumes you are able to apply TGA texture to your model by
yourself, because it is usually simple as drag and drop TGA texture on your model, or super simple material setup in any software.
Since it looks like you have no knowledge about this you have few options
A) model you posted has in its "3D model details" checked "Materials" which means materials should be included.
So you should contact seller directly and request MTL file.
B) Learn basics of some 3D software (Blender is free) and how to import OBJ file, setup basic material and export OBJ file.
C) OBJ and MTL are simple text files and you can create your MTL by yourself in a text editor (Notepad etc.) if you don't
need any special material properties.
Here is simple how to
https://imgur.com/E4IzPOl
More info about OBJ and MTL files
https://en.wikipedia.org/wiki/Wavefront_.obj_file
http://paulbourke.net/dataformats/mtl/