The Key Lime (Citrus aurantifolia Swingle), also referred to as a Mexican Lime, bears a profuse amount of small, thin-skinned, greenish-yellow limes. This juicy lime is known for it's distinctive aroma high-acid content. The image shows the size and color difference between the more common Persian lime and the Key Lime. The average Persian lime weighs about 3 ounces and is about 2" in diameter as opposed to the small Key lime which averages 1 ounce each and is about 1 1/2" in diameter.
If you don't have access to key limes you can substitute equal amounts of the standard, grocery store Persian limes. Lemons are not a good substitute for limes in most cases because the flavor is very different.
While as a rule we don't recommend using bottled lime juice (the type that comes in little plastic limes, for example) it could be used in a pinch in a drink recipe or a recipe where you just want a little more acidity. If you like having key lime juice around then purchase a good version like Nellie & Joe's Key West Lime Juice. That said if you want to make a key lime pie we recommend using fresh key limes or Persian limes.