You can filter products by brand name by adding the brand name variable to the URL:
/default.asp?clientid=[storeid]&f=products_view&d=estore&brandname=Sample Brand

This link will show all products with the specified brand. If you want to narrow by Brand within a category, you must specify the category ID in the URL as well.

The best way to create URLs, would be to browse your website, then navigate to the correct category where the products are displayed on your site. Grab the URL from the browser, then add the variable "&brandname=Sample Brand" to the URL.