Symbols.com - A collection of symbols (mostly western, and both ancient and modern) with a great search function.

The Alchemy Web Site - specifically, their “Graphic signs for alchemical substances, processes, and equipment” subpage.

Symbols in Books of Alchemy - A comprehensive reference collected from a few alchemical texts.  I believe there’s a crappy scan on Wikipedia with information that this page shows in far greater detail.

The Encyclopedia of Diderot & d'Alembert Collaborative Translation Project - While not necessarily about alchemy, it is mentioned several times, and Diderot’s name comes up several times in symbols.com searches as a “definer of terms,” so to speak, on what some symbols represent.  Either way, an awesome and comprehensive reference.

Steganographia, by Johannes Trithemius - A happy accident found during a scavenger hunt for the early “Irrational Treasure” OnDemand release, Trithemius is famous for writing the trilogy of books called the Steganographia, which on the surface is about magic and spirits, but also contains extremely detailed cryptography; the similarities, for those who know the premise of the show, are uncanny.  While there is no direct translation available that I could find (there used to be a partial translation, apparently, but it’s now unavailable), there are several symbols scattered throughout the transcriptions of the trilogy, especially the first book, which might be useful to keep around in case this happy accident ends up being more than just that.