You can find paintings at
Sadar Bazar ( Near Connaught Place)
These are some of the locations I could think off the top of my mind. You can always negotiate at these places and get a painting for as less as 1000 bucks which would adorn your wall.
National Gallery of Modern Art & Delhi Art Gallery.
Dilli Haat is also a good option.
Local markets has some of the best art pieces. You can checkout nearby areas. Sarojini Nagar, Karol Bagh are few of them.
If you want it for home decor then go to Chhatarpur market, there you'll find shops for home decor units.
Someone on Insta maybe. Local talent should be promoted as much as possible. Many top notch young artists on Insta.
National Gallery Of Modern Art.
Kiran Nadar Museum Of Art.
Delhi Art Gallery.
Dhoomimal Art Gallery.
Palette Art Gallery.
Art Motif Gallery.
There are places in champa gali and Shahpur jat and another near indian habitat centre