Not all Tequilas have a cork top. Usually Premium tequila with cork top because that is associated with finest, better liquids. This also makes the bottle of Premium Tequila more expensive. Cork comes from Europe, usually Portugal, but can also come from Spain, Italy or some other mediterranean country. Not every cork can be used, if you just use a regular cork on a bottle of Silver (blanco) Tequila, the cork will eventually start filtrating some light yellow colors to the bottle, so you will have a bottle that looks like a Reposado.
So you have to buy corks that have a special treatment which also make them expensive corks.
