If you are a DIY (Do It Yourself) kind of person, then you can "build" one yourself, without having to solder anything...
I simply purchased a Pulse Width Modulation (PWM) controller on Amazon for $6.99, along with a 12 volt 500 mA power supply from eBay for $3.99. I already had a spare PC fan on which I mounted two rare-earth magnets that I salvaged from an old PC hard-drive. The last thing that I purchased was a 40mm stir bar from Amazon for around $8.00.
The four electrical connections only require a screwdriver, and I mounted everything in a box.
In layman's terms for this application, the "Pulse Width Modulation" controller is something that quickly turns the power on and off. As you turn the dial to the right, the PWM controller simply increases the time that the power is on; as you turn the dial to the left it simply decreases the time that the power is on. In other words, you no longer have to worry about the power supply, or other electrical components, overheating...
I could have bought a cheaper stir bar, but all in all I only spent a total of $19.00 on my stir plate.
In case anyone is interested, here are links to the items that I purchased:
P.S. I am not affiliated with any of these sellers.