Definitely consider Lake Dogs advice about the lard. It's cheap, easy to find in any grocery store (look in the prepared meats isle - bacon, lunch meats, hot dogs, etc.).
Make sure you get the cooker really good and hot . . . THEN apply the lard. Keep it going for a while and apply a second coat after about 20 minutes. Then let the cooker cool down to ambient air temperature. It'll be safe from rust for about one year.
I perform this ritual each spring around the middle of March.
Oh! Also - be sure to heat the cooker up over a place that you do NOT care if grease stains get on it. The first time I did it, I wasn't thinking it all the way through and my wife was not happy about the large grease stain on the patio concrete.