Metal rockers give off a unique style. And though it is hard to get one for "uber-cheap", I do have some ideas for you.
The first thing to do is keep an eye on Amazon's Patio, Lawn & Garden "Markdowns" page. If you are patient and check it from-time-to-time, I am sure you will find a good deal on a metal rocking chair on that page.
This may not be a typical yard sale item, but you can surely find one at the occasional yard, garage, or estate sale.
Certain secondhand stores may be a good hunting ground. Occasionally, your local Goodwill Store surely gets a donation of a metal rocking chair I am sure they would be happy to sell to you at a nice price.
Also, keep a lookout on your local FreeCycle or CraigsList free stuff listings. You may "luck into" a gently used metal rocking chair -- just be aware that this tactic is a long shot.
Of course, we do not always shop with the luxury of time. If you find yourself now in that situation and shopping online is not your thing, I say head out to your local store that sells patio furniture and look for your best deal. For most, I imagine that store would be a Lowe's or Home Depot.
Once you own your new metal rocking chair, I say happy rocking to you!