This is really open ended. The main idea is that the interest rates are always sort of close to accurate of what the standard Fannie Mae interest rates are at that moment. Note these are national interest rates... so there is no zip code or local considerations in this.
One way I see
- find some API with up to date interest rates (suggestions in comments)
- write a script that pulls/scrapes the numbers and puts them into a YAML/JSON file
- make the site read the data from this file
or... just have the front end code actually pull the number on page load, with an appropriate waiting indicator