Marc,
We use
this place for tent repairs. Cost isn't too bad but they can take a few weeks to get a job done.
They might also sell the sealing tape and waterproofing but you'd also get that from a hiking/bushcraft shop. Near the CBD there is Palin and Kathmandu but there are plenty of others around, BCF would probably also have them. I'd also recommend a can of zipper lubricant.