Passion fruit juice is usually found in the marketplace blended with other fruit juices and sometimes sweetened. You rarely find 100% pure passion fruit juice all by itself. These blended/sweetened passion fruit products are also called passion fruit nectar.
The blended products will usually list how much passion fruit juice is used and you usually want to go with the one that has the most in it. I have used Looza Passion Fruit Nectar in the past with good results but Looza discontinued this flavor in 2009. Ceres also makes a passion fruit juice but it has a lot of apple and pear juice in it and tastes more like apple juice to me.
You can also juice your own passion fruits if you want, but it is an expensive mess to do it.
Fear not dear Reader, there is a solution to this problem. Go out and get yourself some frozen passion fruit pulp at a Latin or Asian market. A popular brand is Goya and you are looking for "maracuyá". Just thaw this out out and thin it with water and sweeten with simple syrup to taste. This is a great way to get a 100% passion fruit juice for your cocktails.
If you have tried the pulp let us know how it turned out!