A magnetic stirrer and polarity reversal are different things. Some form of stirring is essential. Whether you purchase or make a magnetic stirrer from a box, fan and potentiometer or PWM fan controller providing you can eliminate any electrical interference or get some form of air bubbler to mix the Colloidal Silver , although I dislike the air bubbler method, a method of mixing is essential.
Polarity reversal does keep the electrodes free from silver oxide ie black crud which can drop into the water.
Take a look at the Bigbear CS generator on ebay because it might meet your needs. Especially as he has now reduced the price back to where it was previously after increasing it to a ridiculous price.
You might be able to make a CS generator using a few circuits such as timer and relay to do the reverse polarity. A box for a magnetic stirrer made from a PC fan which should not cost much to DIY. See youtube for ideas on making a magnetic stirrer. As I mentioned a PWM ie pulse width modulation device to control the fan give greater control than a potentiometer as a potentiometer lowers the voltage and many fans have a minimum voltage so might stop. A very low resistance potentiometer such as 200 ohm or possibly 470 ohm might work. I have an old 470 ohm potentiometer which does the job just about but setting it too low and the fan stops. That should not be a problem with a PWM device as it pulses 12 volts to the fan which should keep it moving at slower speeds whereas a potentiometer will lower the voltage and when the voltage reaches around 6 volts it might stop. http://www.homemade-circuits.com http://www.ebay.co.uk/itm/121760934914
You could get a similar thing direct from china/hong kong for a lower price.