It comes from the compressor section of the engine which us up-stream of the combustion.

No matter how you do it you need to compress air for pressurization. You already have a very large air compressor in the compressor section of the engine so it makes sense to "bleed" high-pressure air from there--bleed air.

The old DC8 used bleed air to drive cabin turbo-compressors, which were just separate compressors driven by the bleed air, that compressed air for pressurization. Too much energy was lost in the process and the compressed air was not high enough pressure to run the air-cycle machines (packs) and required an additional freon vapor-cycle stage. Very inefficient system which was replaced in many of the airplanes in the 1980s.