Amazon has rare earth magnets... Get the size you want... I picked up a two pack of them (18# lift/pull strength) and use them to pull the stir bar from the bottom of the flask. I bought my stirplate, so I didn't need to use them in it.
Find someone's old hard drive. You'll need a small Torx screwdriver to get into it. Or in my case a small pry bar and a vice. They are in the arm piece that reads the discs. There are threads about making DIY stir plates and videos on YouTube. I built one about 2 weeks ago and have already made 2 starters with it. I used a power converter from an old answering machine that I have had sitting in a box for about 10 years.