I buy it from farmers markets or markets, but have to ask them to specially ask the slaughter house to reserve it as they normally throw it away. Usually you (I) have to pay next to nothing, and so end up buy some of their regular meat to keep everyone happy.
It is the fat around the kidneys of the cow, of which there is quite alot! Fat is an important source of calories (not to mention everything else good about raw fat) for the primitive eater. Marrow (in the bones) and hide fat (which is self explainatory) are the other 2 main sources, but suet is the easiest I think to remove when butchering, and so the easiest for people to get.
It lasts a very long time in the fridge (ive had mold growing on it one, but i am 99% sure its actually residual meat on the fat not the actual suet) which is great. People who travel sometimes make a processed food called 'pemmican' using suet (or i suppose any other fat) and dried meat. Check out lex rookers manual on it (awesome), although it is not raw, and more used for travelling or when you can get anything else i suppose (or storage).
I like to warm my suet up a little to kind body temperature in some warmish water, it becomes absolutely deliscious. Other people just take it out of the fridge for a while to take the chill off.
Ermm, all i can think of. People like lex used a ratio of 150g meat a day and 150g of suet I think, which works out at about 600cal pro, 1350cal fat, not sure about %'s but the idea is basically eat a moderate amount of protein and the rest from fat.
Some people also suppliment their diets with wild, seasonal fruit and some other vegetation like samphire.
Anyways do some reading, this site is actually the fount of all things you could possibly want to know!