I would love to help, but it's not my domain. I, unfortunately, witness the side-effects of people stealing them on my day to day. There are many shops in Lisbon that sell them, and there's even a beautiful museum here that it's definitely worth visiting (or signing up for a workshop!): https://museunacionaldoazulejo.gov.pt - it's currently closed for renovation; it should reopen later this year, but that's yet to be confirmed.

When buying - on the streets or in a shop - if it looks old, unless provenance is absolutely clear, please don't.