The phrase "are common only" is correct and usable in written English.
It can be used to specify that something is exclusive to a particular group or situation.
Example: "These species are common only in tropical regions, making them unique to that environment."
Alternatives: "are exclusively found" or "are prevalent solely".
While recalls for local offices -- school boards and town mayors -- are common, only one effort to recall a state's governor has ever been successful.
Although lumbar disc herniations are common, only a small portion of these herniations lead to cauda equina syndrome (CES), which is an uncommon but debilitating disorder.
Archaeocyathans are common only in regions that were positioned in low latitudes during the Cambrian Antarctica, Australia, China, Kazakhstan, Siberia, and North America.
Individuals are common only when nectar plants are flowering well, yet rarely are they common enough during peak flowering to harvest a majority of chuparosa (Justicia californica) or ocotillo (Fouquieria splendens) nectar.
Speeds of 2.5 gigabits per second, enough to handle heavy Net-generated traffic, are common only in the hearts of big cities like New York or Boston, where Verizon runs fiber-optic cable with 48 strands.
However, detecting compound heterozygotes between ΔF508, the most common disease causing mutation, and other mutations which are rare is difficult as some mutations are common only to particular ethnic groups.
But this thread was common only to a point.
Snow is common only in mountainous regions, but frost is frequent in inland valleys in winter.
Mechanical restriction of the growth of the embryo is common only in species that have thick, tough seed coats.
(The word seldom appeared in The Economist before 1986 and began to be common only in the 1990s).
In the movies, of course, hair coloring was common — only characters that were meant to be grandmothers or harridans were gray.
