I get my copper from Julian Adams of Adams Bonsai
http://www.adamsbonsai.com/adamsbonsi2013_008.htm
His prices are fair and an awesome guy! I usually get 2 sets of his introductory specials, and including taxes, priority shipping, it tops out at about $120 and arrives in two days...
I go through a hell of a lot of wire! Due to the fact that I am always wiring up a ton of material, so these two specials last me about a month, month in a half... but for most probably would last the most part of a year if not more.
He sells a really nice wire caddy for copper wire that is hand made out of oak, dove tailed and all, works really well, looks great, and is built to last!
http://www.adamsbonsai.com/adamsbonsi2013_003.htm
Obviously one does not need to use copper... main arguments for doing so are that your trees look nicer, due to the fact that half as thick of copper wire does the same job as aluminum. So, the wire is hardly viable in comparison.
Where I personally have really found the difference besides this... is in it's strength. If one is really doing work with alot of tension, like doing heavy bends and guy wires... the aluminum has a tendency to stretch, especially over time, and in the hot sun that we have here. Also, when twisting the wires to establish the guy wire, aluminum is more capable of breaking, and usually will... not to cool loosing a branch you are trying to bend, because the wire broke and the branch snapped back into place.
You would just have to use thicker aluminum wire... which here again looks a little unsightly.