I'll spare the details and keep it brief: I don't own GTAV. I'm at school with a weak laptop and haven't bought GTAV for my (underpowered) PC at home. But I still want to do some texture design work. I'm majoring in animation, graphic design etc. so I'm all set on software, but for opening car files to extract textures and preview via OpenIV it seems to require an installed copy of GTAV. Is there any way I can make police textures without having the game on this computer? Thanks!

Try downloading a model for a car

Also, use a program like Paint.NET, or GIMP, or similar. The Downloaded Vehicles that Rosebuddy speaks of will often come with a Texture Template which cam be used as a Base Layer (create Decals, Text, etc in another Layer) in your Image Editing Software.

The problem I have is when I download these files, often there isn't a texture.png that has the livery which I can design over. I would have to open the yfts and ytds and whatnot in OpenIV, which doesn't work since OpenIV requires an installed game. 

Bumping this bad boy, hope this doesn't break any rules. Basically my problem is the templates I download have files I'm not allowed to open because OpenIV won't let me run it without scanning for an installed copy of GTAV.