Kegs are avail from all over. You just have to be creative, check scrap yards for the cheap versions, then invest a little sweat and you will have a perfect boil kettle. If the outside looks ruff, you can clean it up with some fine grit wet/dry sand paper. Inside can be maintained with pbw.
One note, when you make the first cut, be aware there may still be pressure inside. Also, Make sure you wear eye protection when doing anything with grinders or any other cutting device on a keg. Should a stainless particle get in your eye you wouldnt like the docs having to get it out.
Sean