Doing a little activism here: please don't buy old ones while visiting Portugal/Spain/etc., as they are commonly stolen goods. Thanks, from Lisbon.
I'll take advantage of your domain knowledge :) Where's the best place to find local artists that produce them?
Meanwhile there are many masters that make these tiles today - I know one personally! - and they all struggle to find clients.
I just googled it and apparently these are often chipped of historical buildings and sold as tourist souvenirs. Thanks for informing us