I have purchased from ebay and amazon.
I don't make any recommendations on where you can purchase them. Research the sellers feedback.
Multiple sellers have the same products.
I will warn that these LED bulbs run hot and are not for continuous usage. A minute or two to back out of the driveway or parking spot is what they work best for.
921 vs new vs older fancy brand
The wattage of the LED bulbs is the equivalent halogen/incandescent output. It's a comparative lighting watt, and not watts consumed.
I have not tested the other socket sizes like 3157, but almost every bulb is available using the Cree xbd LED and should easily trump the reverse stock bulbs.