There are a lot of shops in Ubud selling oil paintings, yet they are becoming less and less, I noted a very nice art shop on Monkey Forest Road, infact there are two, opposite each other, Monkey Forest Road is a one way street, so the shops are at the beginning where you come off Jl Hanoman. The one on the left going down actually paint on site, you could order something to be painted of your choice, take a pic of the ones you saw in Seminyak. Best to walk this road as the shop face is very small and you could miss it if in a car.
The owner of Greenfields Hotel is a painter and had a gallery once but l didn’t see it there last week just opposite the Yoga Barn. You could ask them where the gallery is now.
There are also quite a few artist studios along Jl Nyuh Kuning, towards the Pengosekan end.