It’s a good idea to have a portfolio of your work ready as you grow as an interpreter. Not everyone will know your name or be able to hunt down personal recommendations, and even if they do they still may want to lay eyes and ears on the product that you’re selling yourself as an expert in creating.
The good news is that building a portfolio of interpreting work doesn’t have to be difficult or expensive to be valuable to people who’d like to hire you. I find that even highly-controlled samples can do a good job of showing your skills.
If you’re interested in putting together a work sample for yourself and future clients, I often recommend Joseph Hill’s ASL description of the Black ASL project (YouTube), as his language is in a nice register, and isn’t too fast. Source videos in English are also easily found on YouTube – just pick something you’re comfortable working with.
Modern smartphones are perfect for producing sample videos. For work into English, just set your phone to record video and point the camera at the video of the signer, speaking clearly in the direction of the phone so that it records your voice. For work into ASL, find a video you like of someone speaking in English and play it in the background while you point your phone’s camera to you.
All in all, that these don’t have to be fancy or super-high quality, just good enough to see your skills in action.