A custom site search form

i have a website that is live. It uses open source silverstripe cms framework. its basically an online product catalogue for broadcasting equipment. the products are split into 2 sections 'new and used' and have sub categorys which hold the products. I need a search function on the site that will appear accross all pages in the header which has a normal search field, to the left of that will be a drop down box containing the sections 'new and used'. so the user would choose either new and used then be able to do a search. Depending on what is chosen only that section can be searched.