I know of a few sites, but they're overseas. If you're willing to wait and pay for shipping here's the list I know of : co2art.co.uk ; co2supermarket ; ebay.com ; amazon.com ; greenleafaquariums.com. Though I'm sure if you look around a little, you'll find a place in SA.
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I personally use the 6mm OD clear tubing from Festo for all my CO2 setups. Pro's: 1. I can see if there is any water in the line or not. 2. It is way easier to "hide" clear tubing than Black or Blue 3. and if you use the quick connect fittings it is a breeze to setup. 4. it doesn't leak! 5. it lasts forever - German engineering! Cons: This type of industrial pneumatic tubing is very rigid, and not that easy to work with, difficult to get it over a bubble counter and check valve if you don't soak it in boiling water first regardless of the colour. And it is a lot more expensive than cheap pet shop airline tubing.
If intended for CO2, then ideally you should be looking at a materiel (such as polyurethane (PU) tubing) that is "CO2 resistant", has zero or low permeability and high pressure rating (10bar in the case of PU tubing, for example).