Yep - IMO there's no one place so here's a selection I can recommend.....
Veg/Bark Tan Leather - I order the 8/10 oz Tooling-Holster or the Saddle Skirting - they will split it to whatever size you like if need be...
Wickett & Craig - my favorite
www.wickett-craig.com
Dyes, all types of leather including some exotics, hardware, etc.:
Leather Unltd -
http://www.leatherunltd.com/care/dye/dye.html
Siegel of Ca -
www.siegelofca.com
Leather Factory/Tandy
Mid-Continent Leather Sales Company (800) 926-2061
http://www.midcontinentleather.com/
Linen Thread - 5 cord left or right hand twist is a good all purpose thread:
Campbell- Bosworth:
http://campbell-bosworth.com/catalog/advanced_search_result.php?keywords=linen - I have not used the less expensive Hungarian thread myself so cannot offer an opinion whereas I have used the Barbour's ofr close to 50 years..
Tools: Cheap but OK:
Leather Factory/Tandy - the Craftsman brand
Tools: Better - Osborne and other better quality tools:
Mid-Continent Leather Sales Company (800) 926-2061
http://www.midcontinentleather.com/
Campbell- Bosworth:
http://campbell-bosworth.com/
Tools: Best:
Jeremiah Watt -
www.ranch2arena.com
Expensive but worth it - made by a master saddle maker and a heck of a nice guy!
Gomph-Hackbarth Leather Tools
10754 N. Martineau Road
Elfrida, AZ 85610
520-642-3891
Hackbarthtools@hotmail.com
Robert Beard:
http://robertbeardtools.com/index.html
Gore Tool Route 1
Box 306-B Caddo Mills
Texas 75135-9801
800-859-8338
McMillen Leather Tool Company
864 Four Waters Drive
Sunrise Beach, MO 65079
573-374-7880
Barry King
http://www.barrykingtools.com/
I can recommend all of the above having used them for several years (some several decades) - There are many other suppliers including Hidecrafters. Montana Leather, Oregon Leather, and others including some suppliers on EBay - as always with any unfamiliar suppliers it's caveat emptor.....