Hi Phil,
You can use freshwater aquarium sand, pool filter sand or children sand pit sand. The cheapest would probably be sand pit sand. You can get them from Bunnings Warehouse. If you have a smaller tank and don't require that much sand, you can just get freshwater aquarium sand from most aquarium or pet shops. You would only need enough sand to fill up to a layer 1 inch deep. This is to prevent gas pockets as a result of anaerobic bacteria activity from forming.
The best way to do this, since you already have an axie in the tank, is to remove Charlie and house him temporarily in a container/bucket. You can even pop him in the fridge for a couple of days. This would give you time to change the substrate in the tank. I would recommend you rinse the sand to get rid of fine particulate matter. Thereafter, after removing all the gravel, just replace with the sand. You might like to take this opportunity to unclog your filter, do some interior designing in the tank - placing plants, hiding spots etc. Try leave the slimy layer on your tank surfaces, they are the beneficial bacteria we want to retain. Allow the water to settle perhaps overnight with the filter on. Note that you would need to use a dechlorinator as long as you add any new tap water. You can then pop Charlie into his newly refurbished home.
Cheers.