Cat art works well as oil painting or watercolour, though I prefer oils myself. Unlike dogs that can be made to sit, cat lovers know how independent cats are and getting good source photographs can take some time, though I always get there in the end.
I`m a cat lover myself and so cat commissions are welcome. Besides, cats usually come to me anyway. It`s as if they know who is a "cat person" and who is not.
I prefer to take my own photographs and see the cat in the fur, but it`s not essential or always possible. I can get a better idea of personality with a live sitting and as any cat owner knows, personality is what they want to see in a painting.