What's the problem. If I will create the game for my portfolio. Will people whom I apply to, look into my game art, or it is perfectly legit to use free assets from internet?
Firstly, you should check job listings from actual companies to get an idea of what they want. If they want only to see code samples, you already know you are in the clear.
If you need to send completed projects or demos, of course you will need some art, but if you are focusing this much on the quality of the art it means you are missing one of the most important points: Whistles, bells, and attention to detail that exist regardless of the quality of the art.
One of the samples I sent to get my first job was a Tetris clone. The blocks were 16×16 16-bit style and certainly nothing special as far as art itself goes.
But I added a bunch of little things. One of the blocks looked like a Thwomp from Super Mario World and had a different expression after he landed, and yet another expression when a bunch of blocks were piled on top of him.
Likewise I had explosions for when blocks were destroyed. They were ripped from Final Fantasy VII but scaled down so much no one could tell that.
They also all exploded at slightly different scales and periods of time, adding a lot to the visual appeal.
Other blocks had an occasional sparkle.
Those added nothing to the gameplay and weren’t good art, but they were little touches that told the company, “This person pays attention to the little details.”
So even if you end up having to make your own art, it doesn’t have to be good art to convey a sense of attention to detail, and that is far more important.
But for what it is worth, almost all of the art I use for my test shots in my engine can be found here: Animium
Basically everything there can be used for your own portfolio works if you are showing off your programming (not claiming to have made the models), but do use common sense. Some of the 3D characters are ripped from games and are the actual in-game assets. It would not be wise for example to apply at Square Enix with a demo that shows one of their characters ripped from Final Fantasy XIII.
I would stick with just the vehicles since they are probably actually made by the people at the site.