Hellwig makes an aftermarket set for around $150. They are avialable from most resellers like Jegs or Summit.
Here's one from JCW
1998 - 2004 Ford Ranger Wheel Drive: 2WD SWAY BAR Kit ZX189122T $159.95
You can also try a local junkyard to see if they have any. Some Rangers came with them, so those would be pretty cheap.