Where to get materials and reagents you may ask? Buy for cheap off farmers in bulk or track Auction House lots. Get in touch with most big sellers. Communication is important here, send a whisper or write them a letter. It would be good for both of you to establish business relationships. Constant suppliers and buyers is convenient for both parties to save the time and effort.
Most profitable professions are considered to be Herbalism and Enchanting in pair. You can have Enchanting only for Disenchanting skill, it will bring decent gold. Consider the fact of disenchanting all soulbound armor, not just selling them to a vendor, but I do not recommend to disenchant any kind of weapons, it is more profitable to sell it to a merchant if it is a soulbound item, or sell it on auction house if it is BoE (bind on equip).
Some players choose to take a slightly alternative approach to their gold farming. This approach can take an extensive amount of time and dedication. Many people choose to train a ton of alts, solely for the purpose of making money and multi-tasking. In World of Warcraft Classic, professions are usually paired together. For example, if you’re training alchemy, you will need herbs to create consumables (e.g. flasks) that are used for raiding. An intelligent and observant player could seize this knowledge by creating one alchemy alt and one herbalism alt, swapping items as needed between each other. This fully mitigates the fees by the Auction House, which means your profit is maximised. Another complimentary combination you can use is skinning and tailoring, or cooking and fishing.