Some days ago, I bought 3 piezoelectric transducers, that turned out to be extremely cheap (less than 1€ each), after I found out that contact mics are very easy to make, and who doesn’t want a couple new mics at home?
It is simple, but it helps to have some basic knowledge on electronics.
Watch this great and fun tutorial on that:
Melissa Pons 
I recommend applying a coat or two of enamel paint over the piezo transducers.
This prevents the soldered contacts from breaking out – they are quite fragile otherwise and tend to break after a bit of handling.
Also means you can dip your contact mics in water. – You can get great sounds from water pouring into or draining out of the kitchen sink!
Have fun!
Q. What is the difference between a Schatten Dualie and a Bizzy Bee contact Mic?
Once I had never worked with those, I can only look at the specs but they are so poor in tech detail that I am afraid I cannot help you. They both use large piezos which are supposed to enhance performance on the low region. Besides that obvious info, there’s nothing else I can say.
Thank you.