If you don't want to work for free and are starting from scratch, then I recommend doing the following

- make up sites. Create websites that come from your imagination. These can be riffs off of existing sites if you like but make sure you don't just copy something and change the main pic. They should be entirely of your creation.

- redo an existing site and show a before and after. Include a case study to show how and why you made the changes.

The point of a portfolio is to showcase your work. Whenever I review a candidate, the first thing I do is check that portfolio to see if their level of design is what I'm looking for. Doing the above will show the viewer that you are experienced and competent, even if you don't have a ton of companies under your belt.

If you are already engaging with customers but are losing them because of a weak or non existent portfolio, then spend some time looking over their site before you meet. Put together a plan of what you would do to improve it and walk that customer through this plan. This will help establish you as someone who is not only knowledgeable, but actually vested in your customer's success. Doing this prep work will go a long way towards landing the client. Especially since few people take the time to do so.