In addition to what Cliff said maybe this will help....
Price reflects quality in two ways - where it was tanned (generally either USA or South American/Mexican - there are European tanned hides as well and they are normally premium quality) and the grade. Of the two most commonly tanned hides, USA tanned (Wickett & Craig, or Herman Oak) is normally the best tanning, but you can get good SA/Mex or the old time pit tanned from RJF Leather.
Grade is based on how "clean" the face of the hide is i.e how free of range marks and brands as well as how tight the back or flesh side is.(just as important as a good face.) No hide is totally free of range marks and frankly as long as the hide is not badly scarred I like them as they add "character". Brands can be cut around, but they do take up space, but hides with brands are normally less expensive.
1) IMO don't even bother with bellies. On the chart above cuts F/H and G/I are the bellies.
2) Shoulders/premium shoulders: Same animal basically. Probably the most used Veg-tan leather for sheath/holster making - the most bang for the buck normally due to size and lack of belly leather along the edges. Difference between the two is quality based on grade as explained above. Referring to the chart above: Double shoulders (average 14-16 sq ft) = parts A, B, C, F, & G. Trimmed shoulders have parts A, F, & G trimmed off. A single shoulder is half of a double shoulder.
3) Nature-Tand, Oak Leaf, etc. - proprietary names for various lines of veg-tanned hides.
4) Saddling/Skirting: heavy weight (10+ ounces) veg-tan sides normally used for making saddles. It is finished by the tanneries with a bit more oil so that it retains it's flexibility. It is thicker than necessary for most knife sheath work, although for big Bowies or swords it can be the thing to use.
5) Kip is a hide from a large calf rather than a mature cow. Usually comes in lighter weights only and I have noticed when using it that the grain is usually not quite as tight as cow hide.
6) Sides: The most common cut/size of leather sold average 20-24 sq ft. A side is half of a full hide split longitudinally along the line between B/C, D/E. Trimmed sides have the belly sections cut off.
7) Back or Bends: The cleanest most even (and therefore most expensive) part of a hide consists of sections D & E. Hope this helps.
Carving or stamping requires the same quality hide.
If you can afford it get the best leather possible. For my own use I buy almost exclusively from Wickett & Craig or a new source RF Leathers, who sells shoulders as well as backs - it's really nice leather (old time pit tanned from Portugal) at a decent price.
If you are just starting out, don't have a lot of spare cash, and are unsure of what to get try I'd recommend the single shoulders in 6/7oz weight from Tandy/Leather Factory. They average about 6-8 sq ft and is frequently on sale - it's best though for blades under 6".
For other supplies here's some choices:
Veg/Bark Tan Leather:
Wickett & Craig -
www.wickett-craig.com
I order the 8/10 oz Tooling-Holster or 10/12 oz Skirting and have it split to 8/9 oz - they will split it to whatever size you like if need be...
RJF Leather -
http://rjfleather.com/content/index.php
Very nice double shoulders and sides - these are old style pit tanned hides from Portugal and are VERy nice and reasonable
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
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 for close to 50 years..
Tools: Cheap but OK:
Leather Factory/Tandy - the Craftsman brand
Tools: Better - Osborne and other better quality tools:
Campbell- Bosworth:
http://campbell-bosworth.com/catalog/advanced_search_result.php?keywords=linen
Osborne
https://www.osborneleathertools.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 good suppliers including Hidecrafters. Montana Leather, Oregon Leather, Zack White, Leather Unltd, Springfield leather, and others including some suppliers on EBay - as always with any unfamiliar suppliers it's caveat emptor.....