Good question, Olivia. While we are waiting for people from Cornwall to assist you,this may help, though since I live in Yuma Arizona, USA, I don't get to visit Penzance as often as I would like.
Do you know Lobb's Farm Shop up in St Austell? I know that they sell quince liquor; maybe you could ask if they have the fruit. Their number is still 01726 844411
If you drive up there, I think that it's only about an hour by car, you might want to take a detour at Truro onto M 390 and head NW for a few miles to the Duchy of Cornwall Nursery. They sell quince cuttings and they may have the fruit in their shop. I can't remember the shop's number, I'm afraid, but their main number is 01208 872668 and they will redirect you.
I hope that this helps, and if not, you could always try Google. :p