If your dish calls for green peppercorns and you can’t find them, don’t despair. There are many green peppercorn substitutes that can provide the spicy bite you want in your dish!
Green Peppercorn Substitutes
Green peppercorn substitutes include an unusual range of alternatives that may surprise you. Green peppercorns actually come from the same plant as black peppercorns. They are harvested earlier and dehydrated quickly. You will often find them pickled in brine or salt. Green peppercorn tends to be more tart than the black variety, but it does have some heat. Their flavor lends itself well to very light sauces or fish dishes, where black peppercorns might be too intense of a taste. 
Green peppercorns are used in French and Thai cuisine and are a popular, easy-to-find ingredient in Southeast Asia and India. If you live elsewhere, however, you might have to go to a specialty spice shop to find them. There are a few other spices that will work just as well, such as white peppercorn, capers, pink peppercorns, and green olives among others.
White peppercorns come from the same plant (Piper Negrum) as black and green peppercorns. They are the fully ripened berries of the plant that are soaked to remove the black shells. This removes most of the heat from the berry, leaving behind a peppercorn with a very mild flavor. White peppercorn is a popular way of getting the complexity of the pepper flavor without the heat. This is the closest substitute to a green peppercorn. However, while ground white pepper is easily found, whole white peppercorns can be harder to track down. 
Capers aren’t actually peppers. They’re unopened flower buds that are usually pickled to help remove some of their natural bitterness. Because of this, capers are known for being salty, with tart citrus notes. They work particularly well in fish dishes. 
Capers are the same size and texture as green peppercorns, but they will have more sting in terms of flavor. Use them sparingly, to taste. Too many capers can easily overwhelm a dish with salt.
Pink peppercorns are the fruit of the Peruvian pepper tree and are completely unrelated to green, black, or white peppercorns. That being said, they have a similar delicate flavor to green peppercorns and will work well as a substitution in creamy dishes and sauces. 
Green olives should be a last resort. They have no peppery heat but will deliver the color of green peppercorns, along with some of the briny salt flavor. Use olives with discretion. Chopped green olives may work in certain dishes, specifically savory cooked or baked recipes, but they shouldn’t be used in sauces or in delicate dishes like fish.