Murder is... arguable. I could see circumstances where someone could perform such a ritual where it wouldn't necessarily be murder. Perhaps if you were to subdue and sacrifice someone who was attempting to murder or rob you or someone else, that could be considered vigilantism, or perhaps even self defense (as, despite the name, it doesn't just apply to defending oneself or ones property, but also others and their property).
Alternatively, if one had the connections, you could obtain someone who was already scheduled to be executed. That would be even less dubious, since they already went through what the law of the land considers due process. The theological implications of the sacrificing's effect on the soul of the sacrifice are, of course, troublesome, but it shouldn't be too hard to find someone who was destined for the blackest pits of hell already, especially on death row.
And of course, if your country is at war, you could potentially use captured enemy soldiers, depending on your country's laws regarding the treatment of POWs. But this is a tad more dubious than the other options, and relies on your country being at war, which is far from reliable.