Most thermophilic cultures will be composed of streptococcus thermophilus and lactobacillus delbrueckii subsp. bulgaricus, the combination of which is also known as "Greek yogurt" :-) (This is also known as "Bulgarian Yogurt", which aside for varieties due to location are the same). Given your location, you should have no difficulty sourcing it. You want to use about 15 grams of yogurt per 1 liter of milk.
For Swiss style alpine cheeses (and a few others) you want to get your hands on something with streptococcus thermophilus and lactobacillus delbrueckii subsp. helveticus. That gives part of the "nutty" flavour in the cheese. If it were me, I wouldn't bother going out of my way to get it right away. Just look for opportunities to pick some up. *All* of the big name cheese stores ship to Greece. Yes, shipping is very expensive, but it will be cheaper than shipping to Japan (which is where I live ;-) ). You *can* also culture this from some existing cheese (which I've done a few times), but it's kind of an advanced technique.
For mesophilic, you just need cultured buttermilk. If you have access to raw milk, I would just buy some and let it clabber (leave at room temperature until it thickens into something similar to yogurt). There you have a good mesophilic culture for any cheese. Again you want to use about 15 grams per liter of milk.
You can maintain your cultures pretty much indefinitely. They *will* freeze OK. Just disinfect an ice cube tray, put some in there, freeze it and then put the cubes in a freezer bag. When you want to use it, take out 2 cubes, let them melt, add in about 500 ml of milk and make yogurt from it (around 42 C for thermophilic and room temperature for mesophilic for 12 hours or so). Then use the resulting "mother culture" (also known as "yogurt") in your makes at 15 grams per liter of milk. These will list several months, but I like to refresh them every 2 months.
I have often, in the past, made yogurt in small jars in a water bath with the lids on. Then you can just stick them in the back of the fridge for months as well. Personally, I often find that more convenient than freezing. As long as you don't open the lids, it should be fine. Mesophilic cultures often give off CO2 gas so leave the lid loose while you are actively making it and then close it tightly before you put it in the fridge. I think you are unlikely to make bottle bombs, but better safe than sorry...
Lots of recipes call for "farmhouse cultures" which are essentially a mix of mesophilic and themophilic cultures -- similar to the profile of the major bacteria you would get from raw milk (but still missing lots of other minor players). You have to play with the proportions a bit when using "mother cultures". It depends mostly on the temperature and you cultures. Some thermophilic is quite active at lower temps like 30C, so you can do something like 75% mesophilic and 25% thermophilic. Otherwise you might do 100% mesophilic (i.e. 15 grams per liter) and up to an extra 50% of thermophilic. You really just have to play with it. The same thing with higher temperature makes -- for example, lots of cheeses will do the initial setting of the rennet at low temps like 34 C and then raise it up to 50 C during cooking. So you need a cook proportion of mesophilic in the mix to get good acidification.
It's not as hard as it probably sounds and if you make lots of cheese, you'll get the fee for it. Anyway, if you want to get started and don't want to spend the $50 in shipping for cultures, that's what I would do (and have done quite a bit). However, I think it's worth picking up a few DVI cultures and for a hobby I would recommend buying from some place like cheesemaking.com, even though the shipping is high. Just buy several things at once.
