I am not sure where you are located, but I believe that if you are in the US, you should be able to get the material quite easily if you look in the right place.

I got the following article that may be of use for you:

http://www.kastenmarine.com/alumVSsteel.htm

The bottom line is that it should be easy enough to weld corten to steel, but then you will end up with a weaker structure.

Hope this helps

Regards
Niekie Jooste

I don't have my reference material handy but you will find Corten is either ASTM A242 or A588, depending on thickness. I think thinner materials are usually A242 and thicker (above 3/16") are usually A588. These are "weathering" steels meaning the surfaces rust to form a protective barrier against further corrosion. Your nearest steel supplier should be able to get these for you, or maybe a nearby fab shop may have some in stock.
You should be OK using E7018 electrodes for your welding, whether it is Corten to Corten or Corten to mild steel. There should be enough admixture (alloy pick-up from the base metal) in single pass welds so that the weld metal will have similar weathering characteristics. If you make multipass welds and want to maintain weathering color match, you could make your surface passes with E8018-C3 electrode and E7018 for your roots and fill passes.
If you are going to paint over the repairs, as I suspect you might, the E8018-C3 shouldn't be needed.
Remember that both electrodes are low hydrogen and should be stored at 250 deg F minimum after opening the package. (You can probably get by if you heat them in an oven and then put them into an air tight container, just realize that is a "field expedient" and not according to recommended practices.) If you aren't able to do that, you can still use mild steel electrodes such as E6011, E6013, and E7014. But since you mentioned freezing temperatures, I assume there could be an occasional impact with ice and I would want to use E7018.
If you want to use MIG ( or even FCAW-flux-cored), you can find electrodes equivalent to the SMAW ones I mentioned, but I would look for those with good low temperature impact strength.
CHGuilford