Antonia, have you thought of visiting an indepenent kitchen or bathroom showroom, not just the sheds? They may have tile boards in their showrooms or point you in the right direction!
Alternatively if you have found a manufacturer, contact them direct, tell them where you live, and ask who is their nearest to you, distributor or stockist for smaller quantities. A wholesale distributor should be able to tell you where they supply. We could always order specific metric quantities in UK from our suppliers for our customers kitchens or bathrooms. I can’t guarantee it will be similiar here, but no reason why not.
We bought the tiles here for renovating our house from Castorama, they came in boxes of so many a box, and the number of boxes will be determined by the metric size of wall/floor you are looking to tile, allowing for spares and or breakage, you should be able to buy sufficient no of boxes.