The traditional green gem is emerald, but tsavorite garnet, demantoid garnet, chrome tourmaline, sapphire and chrome diopside are good alternatives. Peridot, more of an olive green, has also become very popular. In addition, green cabochon gems include seraphinite, smithsonite, jade, serpentine, prehnite and more. From this page, you can filter by size, shape, clarity, origin, weight, price and other factors, by using the tools at the top. This will enable you to find the perfect green gems for your jewelry.
I love GemSelect, personally. I browse their site almost daily to see all the new stones available. Their selection is pretty amazing. The quality isn't always the highest and the cut of the stones isn't always perfect. But for the price, and if you look hard enough, you can find a real gem, so to speak.