Autozone. There are 6 in Merida and 2 in Campeche. It wont have motorcycle specific synthetic but they have Mobil 1 and several other full synthetics. Any big city will have 5-10 bike shops that service little work bikes and 1 or 2 that service bigger sport bikes. The big bike shop will usually carry synthetic for motorcycles. Could be as much as 200 pesos/liter.