The SIM cards are under $3 each, and you can order them from your Twilio account that you (should) have already set up.
To find where to order your SIM cards, log in to Twilio. Next, click on the icon that looks like a SIM card. Next, click on Orders. There is a giant red plus sign up at the top. Click that to place your order. Order the number of SIM's you need. The Starter Pack has three; the rest are orders of 10 or more on the next line down. There is no in-between quantity between 3 and 10.
SIMs ship within 2 days. Once you get your SIM's, you'll need to activate them. Go back to Twilio, select the SIM icon, click on SIMs and then click the big red plus button and choose Register SIM. It's simple with visual instructions on what to do. You'll need to select a rate plan. You want to check everything BUT voice. If you need help at that point, let me know and I will assist.
Once registered, you'll need to visit the restaurant and put the new SIM card into the bottom of the Sprinterr. Nothing more is needed and your orders will continue to flow