Size.

The majority of djembes are around 14", however they are not uncommon in the range 10" to 20", larger and smaller may be available.  In any case when purchasing a skin, you should ensiure that you have at least 2 inches of surplus all around, this means that for a 14" djembe, you would purchase a head no smaller than 18".  This surplus is to account for the folding of the skin around the flesh ring and the top ring and to give you enough to grip when first applying the head.

Believe it or not, the part of the world and the conditions the goat was kept in makes a difference to the strength and sound of the skin.  African goats and those from hot, dry places tend to have thin, strong skins, this makes them perfect for a djembe.   European or North American goats tend to be thicker and more 'fleshy' and the skins can be less strong.  As with everything these are generalities and exceptions do occur.

Condition of skin

Raw

The skin may be raw, that is directly off the animal.  If you have a skin in this condition you will need to de-flesh it to remove any fatty deposits & other material attached to the underside of the skin, you may then shave it or you may leave the shaving until the skin is fitted,  the damp skin should be stretched and left to dry in a place away from threat of attack by animals.  You may prefer not to buy your skins like this.  It is only after starting this procedure that you can get an idea of the condition of the skin and can view any scars or other defects .

Prepared

A prepared skin has all the above work carried out and is normally presented to you as a scroll or as a large circular disc, this is my most common experience of buying goatskin (In N Ireland) and the difference in price is nominal.  I pay £15 (us$22) for an 18" skin.

Things to look out for.

Position of the spine: When checking potential skins for purchase, hold them up to a light source. The spine line should run through the centre of the skin.  The skin furthest away from the spine line will be slightly thinner because it comes from the flank or towards the belly of the animal.   This means that the skin should have even variation of thickness on both sides.  A skin with the spine off-centre is normally regarded as less desirable, however some players like the variation in tone that this may bring about.  Skin vendors can cut two flank skins from the same piece of skin they cut one spine skin from and they obviously make more money from two flank skins than one spine skin, but you will find the spine skins (from smaller goats or goats with damaged belly skin or just if you have a good skin-man) if you search through the pile (unless I've been there first :)

Thickness: The skin thickness should be a mirror image on both sides of the spine, a thinner portion will allow more light through with the reverse for thicker parts.  If there is varying thickness when comparing the two sides I put the skin to one side.

Scars and markings: Just as with humans, Goats get cut or damaged and the skin will reflect that.  Scars can rip under tension or at the very least will present an extra-ordinary item to interfere with the skin's vibration when playing.   I immediately reject damaged skins.

Chemicals: Although it is very difficult to tell without asking, try and avoid skins that have been chemically treated with preservatives, hair-remover (lime usually) etc and those that have been salted.  These processes strip the skin of its natural oils and make it drier, more brittle and more likely to break.

Wishlist:   If you can ask or ascertain the part of the animal that the skin came from, then select skins that come from the shoulder of the goat, this is the strongest skin.  You can see this by the pattern of the hair growth even after shaving, the posterior and neck skin look somewhat similar when shaved, but the neck area is thicker.

If you want to buy over the web, try Shorty Palmer http://www.goatskins.com