I've not bought in Greece, so I cannot make a specific recommendation, but Media Strom
have shops in Chania and they custom make mattresses in any size you want, in 2 cm increments. The actual size can vary by ±1 cm from what you order. If you go into Chania on the Omalos road, go right towards the city centre where you join the Kissamos road. Take the first left into a small side street before you get to the next traffic lights at a shoe shop called VoiNoi. There is a Media Strom shop on the corner of the side street. It is usually easy to park close to the shop.
The last mattress I bought is pocket sprung with a layer of memory foam on top. They tend to be more expensive and they are quite deep, i.e. 35 cm, which can be a problem if you already have fitted sheets for a 25 cm deep mattress, but I find it very comfortable.Simba Mattresses
seem to get good reviews, e.g. Which Best Buy. Looking at the construction I am not totally convinced. They also seem to have a limited range of sizes. If one of them suited you, you could get it delivered to Nomad Crete Express and shipped to Chania for around £85.
I know Scooby bought a bed from Coco-Mat on Akritiri and I think he was very disappointed with it. Very expensive and not comfortable at all so I would not recommend them.