I bought my magnetized bases from Litko Aerosystems. They were probably a bit more expensive than what I can find else where, but at the end of the day, the fact that they are already precut makes them completely worth it. For the movement trays I purchased their flexi-steel sheet, which I then cut to fit my movement trays.
They have two strengths of magnets. Standard and Heavy Duty. I got standard for all my 20mm based plastic figs. This is more than strong enough to put them on the movement tray and hold them upside down without any falling off. For all my 20mm based metal figs, and 25mm based figs and cav models I went with the heavy duty. It's not quite strong enough to hold the metal figs on the movement trays while holding them upside down, but it is more than strong enough to keep them from falling over or moving all over the place, which is really what I wanted from them. It holds plastic cav and 25mm based plastic figs just like the standard strength ones one 20mm based plastics. One annoying thing about GW cav bases is that they aren't actually 25mm x 50 mm, even though they say they are. They are closer to 23.5mm x 49mm. I found this out when trying to glue the 25mm x 50mm magnets on to their bases. It makes me realize why it is so hard to rank up lots of the cav models. When spaced properly they actually rank up much nicer.
As someone else said, don't make the mistake of using sheet magnet on both the bases and on the movement trays. The magnet to magnet bond is much stronger, but there are bands of polarity in the sheet magnet and if you don't line them up exactly, the miniatures will not sit where you put them on the movement trays. You can cross the bands of polarity and they will hold, but its so weak, it's not even worth it. You either need to go with metal/rubberized steel on the movement trays and magnet/sheet magnet on the bases or you will have lots of headaches. I found out the hard way when I did this on mine. I've now got four 9 x 11 sheets of flex magnet that I need to cut into squares for bases and I need to buy some more flexi-steel for movement trays. I wish that they had said something about this one their website as I had no idea it was the case.
They are self adhesive, but the connection point is so small on the base bottoms that I dabbed some superglue on the bottom edges of my bases when I put the magnets on the bottoms. I did this to make sure the hold is better as I know that adhesive isn't going to stand up to the forces of the magnets for long with removing and replacing the figs on the movement trays.
