You ask a very interesting question to which there is really no good answer. One of my Ph.D. classes (Physical Organic Chemistry) last semester discussed this question at length and a number of scientific papers written on this subject. There are a few different theories but not one that is dominate and nothing more than conjecture. (I don't know if you are aware but you are essentially asking how life arose.) I feel the best answer is that God created them. You may disagree but there is as of yet really no good theory. If you are asking not where they came from in the beginning but where we get them now then the answer you are looking for is different. Most are purchased or synthesized and derived from previously living material etc. They are important to us for a number of reasons though. An organic compound is anything that contains the chemical element Carbon. This happens to be anything living (or previously living). So in essence organic compounds are the very building blocks of life. The whole of organic chemistry is concerned with the study of organic compounds and their reactions and synthesis. A number of (most all) polymers and plastics are organics.

Once life began on Earth, an active carbon cycle started turning available inorganic carbon into the organic compounds associated with living cells -- especially energy-rich molecules. All life on earth is based on compounds of carbon. Carbon is unique among the chemical elements in its abundance of bonding patterns with other atoms. (Silicon is a contender, but not as flexible as carbon). The carbon was present in the Earth's crust from prebiotic days, the atoms originally forged in a supernova explosion by one or more precursor stars that existed & then blew up before the Sun formed, earlier than 5 billion years ago. (The first stars after the big bang contained only H and He so the sun is at least 2nd generation.) A variety of organic compounds have been discovered (via spectroscopy) in the interstellar medium of deep space -- no mystery there by virtue of the laws of chemistry. Intriguing nonetheless. More controversial is whether impacting bodies (asteroids, comets, meteoric debris) can carry organic compounds (not to mention living cells) to our planet. Contamination at the Earth's surface is always difficult to rule out when searching for extraterrestrial organics. There are space probes out there now, collecting dust samples from space for eventual retrieval & study.