The usual camera repair shops are located at Peninsula Centre, Peninsula Hotel Shopping Centre, Funan areas, you can shop around and ask around
as I always go to Nikon Service Centre, or our Bro David aka fatigue aka part time repair men to help me, so not able to recommend any other repair shops.
but you need to understand that cleaning a lens can be quite costly, and may not be 100% clean, might not justify the cost, sometime it is better to just write it off.
it is always better to buy a dry cabinet to store you camera gears.