I used to make Dowsing Pendulums and sold them at Boot Fairs. Nothing special, I just used Hawthorn and Hazelnut, cut them with a stanley knife, then polished them. Finally I'd put a little eye on the end for the string or chain.
Easy to make - free and you can get some good results.
Saying all that, I've dowsed with my Car Keys, just as good.
Here's a picture, about 8 years old I think
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On the selling side I had a printed sheet of how to Dowse and put them in the little packet.
Richard