Nice how-to ...
Just to add some magnet sources for Europe (Deutsch but I received them without problem in France): http://www.supermagnete.de/
Bonus: They had the habits to add some freebies to your package: gum bears candies and a small sample of other magnets shapes.
And a small addition:
For some parts (support systems, heads, ...), there is no need to add magnets to both the parts.
If you have a strong enough magnet for a small piece, a small patch of steel (from a soda can) is enough to snap the other parts. Beware to choose these can that are in steel rather than aluminium (they are heavier, and you can check in store with a magnet which are in steel)
I used that technique to snap weapons on pathfinders (2mm magnets in the chest shoulders, steel patches on the arms), grey knights weapons (wrists with 3mm magnets, steel patches on the hands), ...
It also have the advantage to not bother yourself with magnet polarity.
One example is my "Shepard" from the ATT 2015 contest (red head in the center, able to swap with the 3 pathfinder weapons):