does anyone know where to buy the circuit board that goes on the belt?
Make one. why buy it when everything to make it is available?
http://www.jairuswhitley.com/files/beltcircuit.pdf
Jairus’ PDF.
Nick-a-tron makes resin belt gizmo kit that is great, check out the for sale section.
making one would be easier. any suggestions on what type of stuff to use.
Some people have used the simple approach of cannibalising a circuit board from something electronic, like a computer graphics card. Obviously you should check size, potential health issues and possible ease of use before settling on any given one in case it gives you a headache later on in life.
Other members of the forum have produced theirs by following the link Charlie has posted, where they can get a photograph of the real board that was used, print it out, stick it to a board and then cut around it, adding various electronic parts where they see fit.
If you’re going for accuracy, then I really think you should get Nick-a-tron’s board. The real ones are often expensive and rare, and aren’t always the correct model, and his is the best replica available.
However the above “cannibal” method is accepted if you’re not going for accuracy.
Where do I get the Nick a tron one from?
Nick-A-Tron, funny enough… 
http://www.gbfans.com/community/viewtopic.php?f=31&t=8283&p=76882#p76882