The answer is yes, you can make your own cymatics device right now for under $20 and I happen to have made instructions for doing just that - if you go to http://www.cymaticsounder.com you can download them for free.
This would also be a great project for kids to do, I made one of these "Cymatic Sounders" for a 4 and a 2 year old and they both loved them.
It's really easy to sing into it and see your voice (even the 2 year old could do it).
Thanks for your question, and enjoy building your first cymatics device!
I am finishing the materials for my cymatics course which will be premiered here at the School of Cymatics in a few weeks from now, and I would love your input to make sure I am including information and resources that would be helpful to you.
Would you take 5 minutes to drop me a line and tell me in a sentence or two what your main interest in cymatics is, and what resource, information, or product related to cymatics would be most helpful to you?