Here is an super EASY way to build one. No need of soldering. Only tool you need is JUST a Screwdriver!
So everything you need is:
Car Audio Tape Cassette Adapter
Audio recorder with microphone input (a computer with microphone input will work fine)
Something magnetic to scratch (credit cards work fine)
Super cheap also. Car Audio Tape Cassette Adapters go for just a few bucks in Amazon or in eBay for example. You can buy it anywhere and any type you want. They are all just as simple as seen in this Instructable. I have made three of these already and all work fine. They have little bit different sound quality, but all work just fine. The ones with stereo are little bit more fun! Remember to make sure that casette adapters "input plug" is right size for your equipment or buy adapter for it.
Cassette adapters are completely "passive electronics" so they do in work both ways. Normally music (electric currence) is inserted in it and magnetic head oscillates. With magneto scratcher it is works completely opposite way. Magnetic head picks the oscillation in magnetic surface and turns it into electric currence.
Step 1: Open up
2) Open casette into two parts
3) Remove magnetic head
YOU ARE DONE! Super easy!