A texture mod is a type of mod so when applied to a track, it would changes only the visuals and not the gameplay

The best place to go is http://alf.red.free.fr/tmmod/
At the top you can toggle the envis and search for mods you want. And when you've found the mod you want just click the dowload icon to get your mod.

Go to C:\Users\YOUR_USERNAME_HERE\Documents\TrackMania\Skins\ and create the following folders:

Alpine (Snow Envi)
Bay
Coast
Island
Rally
Speed (Desert Envi)
Stadium

In all those folders create a folder called Mod
Locate where you downloaded your mod (should be downloads) and proceed to move it to the mod folder for the required envi

Open the game go to make a new track, Hold the CTRL button before clicking it. At the end there should be a new option to use your mod and then after it loads you can make a track with mod :)

Note: You don't have hold CTRL everytime you want to use your mod

Go and load track hold CTRL click on the track you want, clicl edit and the option should be there (you have to hold CTRL everytime)