Yet there are limits. It is considered perfectly good practice to substitute certain items with certain things, but not others. For example, if one were to perform the Return a Lost Lover Spell from Homemade Hoodoo, but one didn't have any access to lavender oil, what would one use instead to substitute for it? The thought might be to use another love magic herb from the Hoodoo Herbs List, such as orris root. However, while this isn't by any means a terrible substitution, it actually misses the significance of the original selection of lavender as an herb ruled by Mercury under planetary magic. In fact, the lavender oil is itself a substitution of an older, harder to find ingredient -- the original versions of the spell were done with liquid mercury, a medically toxic ingredient that is now very difficult to find and which is discouraged from used even if you do happen to find it. Mercury is regarded as a messenger and who acts quickly, and so the spell is probably designed to call upon him or at least influences alike to those which Mercury handles, in order to deliver back home the missing lover or at least provoke communication. Thus, a suitable substitute for this planetary ingredient might be cinnamon oil, which under Agrippa's system would be a Mercury ingredient, and which in hoodoo also has a history of use in love magic.
Understanding the function of an ingredient is key to knowing how to effectively substitute for your spellwork. One must look over the spell and ask: what is the reason for this ingredient's inclusion? No ingredient is there sheerly by chance; if it's included in the transmission of the spell, then it was intended to have a purpose, even if at times the purpose might be obscured by influence of unfamiliar magical systems or by poor repetition of the work. The breakup spell from Zora Neale Hurston is an excellent example of such a case: she probably made a mistake in how she described it, or else was given an incorrect description to begin with. In any event, it seems unlikely she would be "protecting her secrets" by giving deliberately wrong information, over a spell there was no reason she especially had to divulge at all had she wanted for it to remain unknown.
The famous Hand of Glory offers another example of impossible to find ingredients. While a determined person might be able to acquire "the hand of a hanged man" it probably takes more trouble than one would wish, and indeed would probably be a bigger crime to acquire than would be the thefts one uses the Hand to conceal. There are actually quite a few modern attempts at substituting the hand, typically using hand-shaped candles, though no reports are available on how well this actually works. But, one might ask, is it especially the point of the hand to be a hand? Might it be that, rather, the important aspect is that it comes from a hanged man (a criminal)? If that is so, might a pile of graveyard dirt collected from the grave of a criminal do just as well as the base for the candle? The candle itself uses fat from presumably the same hanged man. Might graveyard dirt substitute if worked into the candle, or would it hinder the burning too much? Is the ingredient necessary at all or is it redundant? In the case of the Hand of Glory, it might be necessary to look deeper into its history to understand what origin and meaning were in the choice of ingredients -- or indeed if it was ever real, or just the made-up ramblings of a criminal, extracted under torture where he felt he needed to say something.
Answers to what precisely a spell ingredient might do are countless. An ingredient could be there simply because the spell wants a certain number of ingredients (numbers like 3, 7 and 9 are traditionally thought to hold magical significance). It might be there because of its practical use (a candle can burn with a flame, but so can an oil lamp). Most of the time there's a straightforward traditional use that anyone who is familiar with the work can use to decide -- but even at that, is it based on planetary magic, European folk witchcraft, Solomonic grimoire magic, African tradition, Indian tradition, or any of the other traditions that merged to create hoodoo? The questions must be asked, and you must have capacity to answer them if you are looking to make magical substitutions. This might not be the best choice if you are a beginner: if you don't understand how an ingredient is meant to work in a spell, but you want to omit or alter the ingredient, perhaps instead you're best off looking for another spell to try that will make use of ingredients you actually have on hand, or at least that you can acquire. You might even wish to forego casting such a spell yourself and use, instead, professional spellcasting.