Unfortunately, there is no such an ability in default CS-Cart. Our company, the first-hand developer of CS-Cart Shopping Cart software, offers full range of custom development and web design services and our specialists can build a web store of any complexity for you.
We will be happy to work on your project to turn your website into a dynamic quotation/shopping system with required design and functionality. If you are interested in it, please request a quote, so that we can pre-estimate the task.
Suggested hack :
Use category to list all the postcodes. Assign these categories to vendors. Set up a vendor filter on the left (its already there probably).
So once a user searches for something he can narrow down from postcodes/category on the left.