I picked mine up at Michael's craft store (about $10 with a coupon), and its more than enough for all of my needs. 2 or 3 coats is required to get the magnets to stick well. I also used proper green stuff this time, and it works better than the other 2 part putty I was using. A small amount of ribben did 43 night goblins and 10 spider riders. Super glue of course between layers.
The magnets you see there are the 1/8" x 1/16" from store.miniwargaming.com. $15 gets you 100, so its not too expensive to do whole units.
And of course, everything hold really nicely.