Most artists are willing to create a custom piece of artwork for you, all you need to do is ask! There are different reasons why you might want to ask for a commission, instead of purchasing a painting "off the wall." You might love a certain painting, but it just won't fit the wall you have for it. You may have fond memories of a specific location, person or pet and wish the artist to work from photos you have taken to commemorate a special memory or loved one.
From an interior designer's point of view, you may need specific colours or motifs within the painting to work within a room you are styling.