Check in Building 6&8 at Barwa Village (Near Wakra). they have a lot of options there.

If you are planning right from the scratch, go for a bit bigger.

BTW, I hope you know that you only need MH lamps if you are planning for Corals & 120 Gal tank would be very small for them.

Aqua Art @ salwa road also have alot of equipments and advises as well.

All the best.