Is it possible to download buttons OpenGameArt from the website or do i need extra URL? Seems to be quite complex.
No need extra URL.
You can use the
Advanced Search feature (which is on the left side) to find the picture you want. Pick and click one of the search results and make sure you read the license information. Although all of them are free, some may have their requirements, like attribution, sharing alike (no modification allowed), etc.
You can find the downloadable file, which is usually at the end of the information. For example this
BubbleButtons.zip in the red circle in the image below: