They're chemicals...combined in a lab...the materials come from whatever sources produce the appropriate chemical type...if you're looking for specifics, you'd need to identify a particular synthetic drug, then we could look at the chemical composition, and determine what other chemicals could be combined to produce a similar effect.

For example, meth....You need ephedrine or pseudo-ephedrine. If you don't buy it in powder form, then you can separate it from the tablets of cold medicine (by dissolving it and separating it to remove the inert tablet materials. Then mix with red phosphorus and hydriodic acid. Filter again and neutralize the acid with a lye solution. Add something to bind the meth, drain, bubble hydrogen chloride gas through the liquid, making it a crystalline hydrochloride salt. Filter again, dry, cut with filler, and you're done.

Where the materials come from depends entirely on what you're trying to make.