If the product you are wanting to purchase is currently out of stock, there may be an alternative.
Many RAM kits are sold as both kits and singles, and are usually exactly the same product, just packaged differently.
To check this, go to the product page on the ramcity.com.au which has the kit you are looking for, then scroll down to the product description section, where additional capacities and kits are shown. (See the below example video.)
Here you can see that Crucial part CT2K8G3S1339M (2x 8GB RAM kit) is out of stock currently. Scroll down the page and you can see this product is sold in a single package as well, which is in stock. (Therefore - two units of CT8G3S1339M is the same as one unit of CT2K8G3S1339M).