Most hardware stores should have it, such as Mitre10 or Bunnings, just make sure you read the label properly, it should say safe for aquariums, if it doesn't, don't risk it no mater what the salesperson says.
Do not get one with mould inhibitors in it either, this is usually sold as bathroom sealant.
The one I have at the moment is Fosroc Silaflex RTV, but there are other brands.