Sichuan Peppercorns were banned in the U.S. from 1968 to 2005 as a possible carrier of citrus canker but real enforcement didn't start until 2002 - then supplies started to dry up. They are now legally available again and in good supply. Current regulations require heating the peppercorns to 160°F/70°C after which they will be somewhat less red in color. There has been debate about how much this affects the flavor, but Sichuan recipes generally call for them to be toasted before use anyway, so this debate seems irrelevant.

The empty fruit shells are much used in Sichuan China and in the Himalayan region (Nepal, Tibet, Bhutan). In Japan the leaves (kinomie) are also used as a flavoring for soups and vegetables. The leaves are said to have a flavor somewhere between lime and mint. In general the fruits are dry roasted and then ground before adding to the recipe, usually near the end of cooking.

Subst: There is no real substitute, but if you can't get them use this formula: Grind together 1/2 t black peppercorns and 2 t coriander seeds. Add grated zest of 2/3 lemon (yellow only). This will fill in for about an equal measure of whole sichuan peppercorns. While it lacks the important numbing effect it will fill in for the flavor.

Varieties of this seasoning:
China:   Shanjiao, Jaio, Fajiu (Canton) (Z. piperitum), used mostly in Sichuan provence.
Nepal:   timur (Z. alatum) is a species important to the cooking of Nepal and Tibet. It's flavor differs a bit from the Chinese in being less citrusy and more spicy but it's pretty much unavailable in the U.S. so use Chinese.
Korea:   sancho (Z. schinifolium) is used in Korea. It is smaller than Chinese, green in color and has little pungency. Koreans also use chopi (Z. sansho - see Japan).
Japan:   sansh, sansho (Z. sansho) is though by botanists to be the same as the Chinese Z. piperitum.
India:   tirphal, tilfda, tippal (Z. rhetsa), is much larger than the others and green in color.
Indonesia:   andaliman (Z. acanthopodium) is smaller than the others and often sold as clusters. It is less pungent than the Chinese and very citrusy.
Tibet:   g.yer ma (Z. alatum) one of the few spices available in that region, used with yak meat and innards.
U.S.   prickly ash (Z. americanum) has not been used as a seasoning but its anasthetic properties have been applied in toothache potions.
U.S.   fagara (Z. piperitum) a market name for Sichuan peppercorns, apparently a corruption of the Cantonese Fajiu (most Chinese immigrants in the 19th and 20th centuries were from the Canton area).