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I'm getting back into 40K and my wonderful girlfriend got me a nice squad of Killa Kans for my birthday. I plan on scratch building a Grotzooka for it, but decided I wanted to be able to switch it's ranged weapon from game to game. Sadly I had no idea how to make a modular weapon system. I went a googling and somebody on this site mentioned using magnets. So my main questions are, where to get magnets for this sort of thing, how strong do they have to be to hold Kan weapons in place, and does anybody have pictures they could show me of a successful magnet job (preferably on Kans, but other units will do as well)?

Since you're in the US, K&J Magnetics is a great source, if you want to shop online - decent prices, reasonable shipping, and they carry pretty much anything you could ever possibly want. Magnets get cheaper when bought in bulk, though, so if you only want a handful for this one project, just go to a craft store and get a pack of rare earth magnets. You'll pay far more per magnet, but won't burn money on shipping or extras you won't necessarily use.

Neodymium magnets, grade N42 (the ubiquitous "rare earth" magnet) is the standard, and has rather good pull. Conventional wisdom says that the biggest magnet you can fit in the joint is the best choice, but projecting parts, as one finds on a Kan, are problematic - torque becomes an issue, and shearing force isn't resisted as well with these magnets as pull. To keep the parts from rotating, I recommend combining a smaller magnet with a pin (glued on one side of the joint, only). The pin supports the weight, while the magnet keeps the parts from falling off or drooping excessively.

I've found 1/8"x1/16" disks to be both small enough (when countersunk) and strong enough (I use multiples on really big jobs, or would supplement with a pin, as mentioned) for most tasks, but I also tend to treat magnets like glue in woodworking - an adjunct to strengthen a physical joint, not the bond, itself. Everyone has their own preference.

As for direct tutorials, nothing comes to mind, but there's a more general vehicle magnetizing guide in the Dakka articles section (one of several, actually, ranging from Valkyries to Terminators) to give you the gist of the process.

Magnetising will be a lot easier (especially re torque, as oadie says above) if you mount the weapon directly on the 'shoulder', i.e. don't use the 'arm' at all. Personally I think it looks better too.