I suggest if you're trying to build a portfolio and aren't working as a copywriter at the moment you create 'conceptual work'. This is when you choose a product that already has marketing materials and re-do an ad or whatever in your own writing.
Though it takes work and you shouldn't represent yourself as having been paid for it. Creating conceptual work is a great way to start building a portfolio that you can use to get paying jobs.
Hope this helps,