You buy these in different sized sets- 100, 300, 500, 750. We started with the 300 set and upgraded to the 500. It might be time for another upgrade to 750. Also there is a computer interface where the kids can watch the electrical impulses on an oscilloscope. They used to sell it separately so I need to see if we can still do that.
They are all built on this base and the pieces are color coded. the pieces literally snap onto each other so there's always a good connection. The tricky part is building the circuit correctly. It has a place for the battery packs and you can switch them out easily, but I noticed the other day they sell an AC adapter for the sets. That might be worth getting.
You can find these at Amazon and other places on line I'm sure. These were a gift from my in-laws, but Rainbow Resources carries a full line.
Snap Circuits are a great learning tool and a lot of fun!