I believe most of us buy salt mix from the pet store/fish store/aquarium store. It is especially made for aquariums, and contains the correct mix of trace elements.
Some marine research centers here do provide treated seawater that's available, and some fish stores also sell treated seawater. I have no idea what is available to you in Colombia. If you're near the ocean, it's possible you could collect your own, if it's not too polluted.