I use Nicotine River a lot for many things, am particularly fond of their nic salts.
It's really hard to recommend flavor sizes, if it's hard to get them where you are. I personally started with small, tester sizes, but if it's going to be harder to get flavors, I would consider 30 ml or so, depending on how confident you are that you will like a flavor. Even today, I still try testers first, with new flavors (some of the time) but I don't really know what to recommend in your circumstance, possibly something bigger but you will then run the risk of not liking a flavor you've bought in a larger quantity, etc.
Anna