Where the ingredients come from
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Sourcing in skincare is a documentation exercise as much as a purchasing one, because what you can claim and prove depends entirely on what your supplier can tell you.
What you are actually buying
Four categories with very different requirements. Water, which is not inert. Oils and butters, which have their own shelf lives and go rancid. Functional ingredients: emulsifiers, thickeners, preservatives, chelators, which need specifications. And actives, which need specifications, stability data and usually a price that makes you think.
The documents that must come with it
The INCI name, for your label. Usage range and pH range, so you know where it works. Solubility, so you know which phase. Heat sensitivity, so you know when in the cool-down it goes in. A certificate of analysis for the lot. And a safety data sheet.
A supplier who cannot provide those is selling you an ingredient you cannot label correctly, cannot formulate responsibly with, and cannot substantiate.
Water is an ingredient
It is usually the largest component by weight and it carries a microbial load. Tap water is not suitable; distilled or deionized water of appropriate quality is, and it should be fresh rather than standing open in the workshop.
It is also the phase everything dangerous happens in, which makes it the one people think least about and should think most about.
Buying quantities that make sense
Raw materials have shelf lives. Unrefined oils go rancid in months, actives degrade, and a year's supply bought at a discount is a year of decline.
Buy oils and actives small and often, in dark sealed containers stored cool. Waxes, butters and most emulsifiers tolerate quantity.
And remember that a new supplier for an existing ingredient is a reformulation rather than a purchase, because trace differences move pH, emulsion behavior and preservative efficacy in ways that surface later.