FINDSTONE is a nexus of contemporary sources in the commercial stone industry.
One of the most useful aspects of this site is a searchable database of images of many different types of commercially-quarried stones throughout the world.
This site is for a sculpture supplier in the United Kingdom which forges their own line of chisels, including mallet-head chisels that are so ideal for carving soft stone.
  The Compleat Sculptor

At this New York supplier you will find a wide  selection of sculptor's tools, supplies, and  carving  stone, including imported mallet-headed chisels.
These folks carry tools and supplies for the stone industry.
If you want to go into the stone floor polishing/countertop 
 making end of things they've got it all -- though they do 
 have some stuff a sculptor might just use as well.
Several warehouses located in North America, including Atlanta.
Don Dougan   -   sculptor
are the folks that invented the pneumatic carving hammer over 100 years ago and continue to provide
top-of-the-line tools for the stone carver.
Joseph and Sons 
When next in Paris you should 
stop by this tool supplier across from 
Pere Lachaise cemetery and look at 
their selection of stone working tools - 
especially the traditional hand-forged 
mallethead and wood handled chisels 
for carving soft limestones.
Located in New York,  SCULPTURE HOUSE
has imported and sold sculpture supplies by catalog for many decades - their 'house brand' tools are in turn available from many other suppliers across the country.  
"Books you thought you'd never find . . . " 

A book is an essential tool, and when it is out-of-print Alibis' network of booksellers helps you find it and then handles the sale. 
To say their specialty is woodworking is an understatement, but this unusual neighborhood hardware store in Atlanta carries a wide variety of tools that any sculptor might find useful.  They have built a nationwide reputation as a mail-order woodworking supplier, and they provide numerous hands-on masterclasses in woodworking-related subjects. 

This stone carving supplier in Rome (with a shop in Carrara as well) is one of the few places you can still find a traditional Bow Drill, in addition to a complete line of Italian-made handtools and pneumatic hammers.  They also carry powertools and supplies for working mosaics.  
The website is in both Italian and English.
The list of suppliers below is far from complete, but these are all companies with whom I personally have done business with over the years.  I have found that all of these firms are reputable and each one offers something unique.  However, as death and taxes are the only things that don't change, caveat emptor.

  Note: the listings are in no particular order other than what          
simply looks good on the webpage when I add a new listing.
Though they specialize in kits and videos for lifecasting, these folks also carry all types of other moldmaking and casting supplies for the sculptor.
G.GIBSON & CO. sells top-quality tools for the stone masonry trade and stone carvers.  They manufacture carbide-tipped and high-carbon tool steel carving chisels (hammer-head, mallet-head, and pneumatic-hammer shank), a variety of hand hammers and mallets/dummys (steel and brass), and scutch combs.  They also carry a wide range of Renzo Milani riffler rasps, the entire range of Cuturi pneumatic hammers, and the Gelma 'Woodpecker' electro-mechanical hammer.  Located in the UK, they ship worldwide.
This stone tool supplier is located on the Mediterranean coast of Spain and carries a complete line of hand and power tools for working stone. The website can be viewed in Spanish, French or English.  Though I have never bought tools from them I am part of their online social sculpture community.