The following is modified from my Amazon review of Snap Circuits (SnapCirtuits) -- a highly recommended electronics toy (a review by Mr. Verwillow helped sort out the confusing supporting web site story):
... The toy is the work of genius, but the manuals could use a refresh, the web sites are the work of a pre-Internet generation (The company was founded 30 years ago by two electrical engineers), and marketing is not exactly a priority.
As best I can tell the official web sites are www.snapcircuits.net and www.elenco.com. (www.snapcircuits.com now redirects to www.snapcircuits.net). Elenco.com tries to cover their educational market, soldering kits, and non-toy devices, so the Snap Circuits (SnapCircuits) site is a better bet. is your best bet.
The web site parts page includes upgrades kit from the simpler to more comprehensive Snap Circuits kit. I think the SC-500 is the sweet spot for most of us.
I strongly recommend the inexpensive Student Guide (experiments up to SC-300) and the Student Guide extension (covers the added experiments of the SC-500). Some Elenco documents mention a teacher's manual, but I get the sense that's obsolete.
I had to print out a paper form to order these, and even do my own arithmetic! First time I've done that in over 10 years. There is a (third) online site for ordering the manuals and other parts but it was so poorly done I preferred to trust the post office.
This toy really is a blast from the past, but it's wonderful.