There is a specialist aquarium shop on Salwa road. I've never seen them selling round tanks but they may be able to help you import one.

To reach the shop, drive outbound on Salwa road until you reach the junction with Hardee's, Tata trucks etc (it's the lights after the wholesale market junction). Take a u-turn and on your right you'll see a shopping center with QNB, QTel etc - it is the very first set of shops you will see after you make the u-turn. The aquarium shop is the last shop on the strip, closest to the traffic lights.