What is the liquid used in instant shoe shiner products? Ive tried refilling it with WD-40 but I would like to know if there are any cheap alternatives to refill it?
It's silicone oil.
You can use a mineral oil. You can find it as baby oil and if you don't like the odor, also can be found as fleet enema products (eg. http://fleetlabs.com/fleet_enema_products.php). For larger and cheaper amounts, you can buy it in computer hardware store, farm supply store, or even in vet clinic.