Look in the OS X App Store. Search for bar code. You want an affordable application that has been updated in the last six months, has good reviews, and supports Intelligent Mail bar codes. Watch out for those that provide a long list of supported bar codes, and make you do an in-app purchase to get the bar code you need.
An example is iBarcoder Free. If you download the free app, the first thing it does is pop the in-App purchase panel. But none of the $15 categories informs you which provide Intelligent Mail support. The basic application appears to provide this bar code support until you go to generate it. Postnet works just fine without additional charge. One positive is that you are provided with several output image formats and can specify up to 4900 dpi. The full version is $50USD as iBarcoder.