Unfortunately, not everyone uses the same set of symbols, though that is changing. Do you have access to Autocadd? They have quite a library with explanations. You might also check with ASME and/or the AIChE web sites. Also would recommend you check out the Process Industry Practices website at
though their focus is not exactly in line with your question. I can't say for sure who gives it away and who sells it, but you should find a usable table at one of these sites. There's a good article at
P&ID symbology for both process and instrumentation and guidance on their use can be found in two British Standards, BS 1646 & BS 1553, which form the basis of symbology in th eUK and Europe.