The cheapest route is to go with a universal radiator from some place like summit racing. As long as it has the inlet and outlet on the same sides as your factory radiator. You will just have to fab some brackets up to make it fit.
A nice aluminum radiator can be had from some place like summit, jegs, or any classic mustang performance dealer. If you want one that drops right in, plan on spending close to $500 or so. Price will depend on if you have an auto or manual transmission.