What is the liquid used in instant shoe shiner?
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.