The Shield right half is an item which can be bought in the Legends' Guild for 750,000 coins. However, it is cheaper to buy the combined shield from the Grand Exchange, rather than buying one half and searching for the other. It can be combined with the shield left half (provided that you have 60 Smithing) to form the dragon square shield, one of the strongest shields in the game.
Trivia
Previously the right and left halves appeared to be two parts of a kite shield. Jagex later changed this.
Despite the shield being split into a left and right half, the halves appear to be top and bottom.
The shield halves have not been graphically updated to look like the completed shield.
The Alchemy values of the right half is actually higher than that of the completed shield (450k vs 300k for High Alchemy).