Wall art can be in many forms.
Personally I like the idea of a feature wall. The feature could be the entire wall, or part of the wall.
Here are some ideas for wall art:
1. If going for pictures, then buy high quality prints that are on acrylic or glass. You can get much better resolution on these materials compared to canvas, meaning the artwork will stand out more.
2. Fit natural materials to your wall, such as wood or slate. Or you can even buy
PVC sheet with a 3D slate effect, as shown on this website.
3. Decorate your room with grey colours, but have a feature wall with some vibrant colours. This can really create the wow effect.
4. Add decorated sculptures to your wall. For example, you could have a decal of a tree, but you can place a 3D looking bird sculpture on one of the leaves to create the illusion of realism.
5. In general I would say Wayfair and Amazon are very good stores for all kinds of wall art.
6. If you have a good imagination, then it don't have to cost a lot!
Good luck!