I was looking for a cheap source of Mg and i found that some sacraficial anode rods from water heaters are made from Mg. From what i understand the rod corodes in place of the tank. So if you bought one, or obtained some from old broken water heaters, would it be practical to grind it down and if so how much Mg would you obtain?

Actually most modern day sacrificial anodes in hot water systems are alloys of Zinc, Magnesium, Aluminium and trace amounts of other more expensive metals. The only ones I have seen containing only one metal are made of zinc.

Getting them out of the old water systems not very efficient as they are designed to corrode in place of less reactive metals like Iron using an electric current (Cathodic protection).

If you want Magnesium, look for rare metal scrap yards, metal dealers, shipwrights and industrial marine supplies. It should only take a few phone calls to find a source. Don't be lazy.

Not exactly too cheap, but I found a reliable source for small quantities at my local Army/Navy surplus store. They sell these "Quick Firestarter Kits" that are a block of magnesium (3/8"x4"x1.5") with a strip of flint embedded lengthwise. All you need do is scrape a few slivers off with a knife in a pile and cover with some twigs, then scrape the knife down the flint and there you have it. A nice fire, windy,wet, or not. (Or grind the sonofabitch up and do what you will with it!). $8.95 US.

I gave up looking for old "mag" wheels a long time ago.

Oh, yes... firestarters... Many people are using this as Mg source - including me!
But you can also buy some magnesium ribbon and shove it down - it appeared to be more money friendly :) .

If you're in the US or Canada (and I know this excludes many members) then one of the best places you'll find is http://www.magnesiumsales.com/index.html . Compare that price to a firestarter bar which is roughly 40-50 grams.

K9, I've bought magnesium from the link you supplied. The prices are very
reasonable in any quantity, especially on the higher end. It comes in blocks
or sections of a block depending on how much you order. It grinds up easily
on a bench grinder. Be careful though, too much friction from the grinding
wheel could cause it to ignite. The Mg in course grade is more flammable
than my finest powdered aluminum.

Grinding magnesium is a very hazardous process because of the inevitable dust build up. One friction spark or a magnesium particle getting sucked into the air vent of your grinding machine...BOOM.

There's a vid somewhere that shows a dust explosion of 1g of aluminium. Take a look at it.

Hows this for an Mg source:

Vifa make loudspeakers. The basket on the 10" woofer is made of Mg. So when your 250W amplifier blows up and toasts your Vifa woofer, save the basket for the Mg. There's probably a good 100g or so there, for only US$60! And you get a free magnet too!

Guess who is speaking from experience ...

If you want to grind block of metal, you can think of http://members.shaw.ca/justin.le/grinder/grinder.html (a metal grinder)
It'a a machine wich grind any metal with sandpaper.
A friend of my build one like this, and it works very good! the yield is nog that high, but it can run day and night. It took about 6 hour to get 30-40 gram. He used sandpaper mesh 320 to obtain very, very fine powder wich gave high quality flash if mixed with KClO
In order to prevent a dust explosion, white spirit is dripped on the sandpaper.

In a boatshop you can buy Mg anodes, they're not that exensive - 1,3 Kilo for ~17,5 $ I paid

I hope I helped you a little,
greets,

COB