Must I take the one goods cube available on a specific site
May I take two goods cubes even though they are a different sites on the same island?
I'm not sure what you meant by the first question. The general rules are:
1) You can only buy the goods that are available (ie, on sites on the map board). Each site produces one good each winter -- and when it's gone, it's gone (at least for trade purposes) until the following winter.
2) Your ships can only buy from sites with which they share a space. You can buy from several sites on the same island, as long as you have at least one ship there, and you can buy from several islands as long as you have a ship at each island.
3) The number of goods you can buy limits your total purchase (ie, two goods total, not per ship). But you can share that limit between your ships as you see fit. (Note that this becomes more important if you use an advisor that increases the number of goods you can buy.)
4) Goods go into the hold of any ship in the space in which they are bought, space permitting (a ship can only carry goods up to it's hold value). You can dump cargo to make room.
Hopefully something that one of us said answers your question!