The one thing I find about Garden Centres on Crete is that they are quite limited in what they offer.
I have lived here for 18 years and enjoy my gardening and am always looking for something different.
I use various places, including ebay. I have had plants delivered from Spain, mainland and the canaries, Portugal, UK, Gemany and many from Holland.
For plants I like http://www.myplantshop.eu
. For something really different I have used http://www.canarius.com/en/
A bit pricey but some of their plants are impossible to find anywhere else.
I find the Trachycarpus Wagnerianus Palm almost bullet proof. Not too large, around 6M max, and copes with the south winds extremely well. Also I find that the Dracaena Draco (Dragon Tree), the national tree of Tenerife thrives here, but, I never find it for sale here. I grew one from seed that I kept in a pot for 10 years then put in the ground 6 years ago. It absolutely took off.
Also, Trithrinax acanthocoma is a lovely hardy palm that prefers alkaline soil, unfortunately again, I haven't found for sale here. I bought from Holland.
Keep in mind that Crete has alkaline soil and some plants can suffer from Chlorosis as iron is more difficult to absorb in alkaline soil. Chelated iron can help greatly with this problem.
The 2 main problems I have found is south winds, very devestating. Cold wet winters. Heavy wet clay soils in winter will kill a lot of tropical plants. I have successfuly grown some tender plumerias (Alba, Pudica, Sercifolia, Thai, Australian etc) by amending the soil. I dig a large hole, fill with Coco Peat and lava rock or perlite and plant them in this. This gets them through the winter without root rot.
Seeds are a wonderful way of getting the unusual. Many of my plants were started from seeds. There are many options available for seeds, but, I find http://www.rarepalmseeds.com
I have grown some wonderful aloes from seed that I don't find here. Aloe Plicatilis is one of my favourites, thrives here. Most aloes will thrive here, but, few are offered for sale. You can also find on ebay. Aloes, Adeniums, Plumerias can all be posted bare rooted without any problem. They will soon spring back into life.
Also, there are some really tropical looking yuccas that will thrive here. Yucca Rostrata, Brevifolia (Joshua Tree) and many more.
If you truly want something different, you will have to look further than the garden centres on Crete.