I wonder if there is a forum where people who are interested in game making, but have different skill sets can meet and exchange art and ideas.
For example: I’d like to model cool things, but can’t make good looking concept arts by myself. So if there was a forum , library etc of freely usable concepts I’d glady try modelling then send the model to the concept artist if he/she would like so. I’d like this way much better than to search the web and ask permnissions. I think there would be better chance that the art then could be used in games.
I don’t know of any forum that matches that description. But PolyCount seems like the hangout for 3D game artists. Sorry if you already know it 
Try www.CGhub.com
Most artists wouldn’t mind if you use their concepts as reference, as long as you give them proper credit afterwards. Of course you can ask them if you’re unsure 
Edit:
If you want to use the models in a game and intend to sell that, you should ask the concept artists definitely for permission, or negotiate something with them.
Thanks, I will look into those.
The basic idea is just to practice… but then who knows… i might get inspired and better to know beforehand if i could use those arts.
If you want concept art, look here: http://www.conceptroot.com/concept-art-gallery/
Speaking of concept art and Polycount…
There’s a thread there currently that has a several gigabyte zip file full of thousands of concept and reference images. I haven’t looked at all of them, and the images are mostly (or entirely) unattributed - BUT, you can use a reverse image search engine like Tineye to find the author and ask for permission.