Where can I find a replacement seat unit liner for a turquoise Tambero?
Just so, you can't. You need to contact the seller of your pram, or directly the manufacturer. They should be able to sell you the spare parts you need if you insist on an original one.
If not, an answer would be - either by having a second-hand piece and use it for parts or find a tailor to make one for you. And of course, a universal seat liner is always an option - just google 'universal stroller seat liner' and you'll be good to go ;-)