Wampee

Find - Wampee can be difficult to find in stores - especially outside of Southeast Asia.  This is further compounded by the fact that wampee is in season for a relatively short period of time, and the fact that the fruits are somewhat perishable.  Still, the fruits can sometimes be found at large and well-stocked Chinese grocery stores, especially in communities with large Hong Konger communities.  I've encountered them at T&T Markets in Western Canada, for example.

If you live in a tropical or subtropical part of the world, there's a chance that you might be able to find wampee growing near you (or that you could grow it yourself).  Wampee is commonly cultivated in Southern China, Hong Kong, Taiwan, Malaysia, Vietnam, Thailand, the Philippines, and Indonesia.  It's also sometimes grown in Northern Australia, Hawaii, Florida, Central America, India, and Sri Lanka. Various cultivars are often sold online by tropical plant nurseries and greenhouses, and seems to be popular for its hardiness, edibility, and attractive appearance.

Dried wampee fruit (and roots) can often be found at Chinese herbalist shops. For more on these, see the substitution notes in the Culinary Info section below.

Choose - If you have the opportunity to taste wampee fruit before buying it, take it.  It's easy to tell whether or not you're getting particularly sweet or particularly sour fruits, and this can greatly affect how you use the fruit.

Substitutions - It's difficult to substitute for wampee fruit in a direct sense, as the flavour is fairly unique.  Texturally, similar fruits like lychee and longan can work, though they have characteristics entirely of their own.  Adding some lemon juice or another sour fruit to the mixture can help to capture some of the sweet/sour characteristic that's typically encountered with wampee.  The seeds, bark, and roots all have their own unique herbal characteristics that are difficult to replicate.  I have had success using the skins from the fruit to add some of the camphoric/medicinal flavour that you would otherwise get from the leaves (e.g. when steaming fish or stuffing chicken).  While this isn't a direct substitute, it does add depth and complexity to dishes using wampee that can otherwise be difficult to obtain with access to the other parts of the tree.

Varieties & Related Species

Varieties

A number of named varieties taken from a translated 1936 work are often mentioned online,¹ but these are by no means the only varieties that exist, and may (or may not) overlap with other named varieties in China.  Broadly speaking, wampee can be divided into a few categories based on the primary flavour characteristics (sweet vs. sour) and the shape (round vs. oval/elongated).  Within each category, there exist varieties with primarily sweet or sour fruit, and with seeds of varying sizes.  "Chicken Heart" Wampee (鸡心黄皮), so named because of it's shape, is a particularly popular and important variety with large fruits and a generally very good taste.²

Sweet wampee varieties are generally preferred for eating fresh, while sour varieties are more popular for cooking and making preserves.

Related Species

Pink Wampee - Clausena excavata
A small, pink, thin-skinned wampee variety.  The attractive fruits are quite small, which means that the large central seed leaves very little edible flesh.  They're also often quite sour.  The leaves and other parts of the tree can be used like wampee or curry leaves.

Curry Tree - Murraya koenigii
A very important food plant in much of South Asia, the leaves of the curry tree can be used in much the same was as those of the wampee tree.  The fruit of the curry tree is quite dissimilar however, with purple skin, dark flesh, and a different flavour.  The seeds of the curry tree are also toxic, and shouldn't be consumed.

Footnotes.

1. Wampee Varieties - Purdue University
2. Clausena lansium - Scientific Database of China Plant Species (in Chinese)