I would recommend you go to the area near the fish market & vegetable souq. There are many garden centres there. One after the other. Check them all out first as there is a variety of prices. Also in the Omani souq they have plants, maybe more variety, but more expensive.
How to get there? Go from Qatar Decoration round about towards Midmac. Go immediately onto the slip road and turn in to the street behind Bukhanan furniture.