There are many internet forums on sustainability, there is a good chance there is one in your country. I'd recommend googling for it
Another way I personally connected with other sustainable thinkers in the past is by doing online courses on the topic. For example both coursera.org and edx.org have courses on sustainability and environmental science and those courses usually have discussion assignments where you can react to other people's posts
Is there a particular sustainability problem you are interested in?
No problem @VladIlie. Renewable energy isn't my strong point, EnergyNumbers probably knows more, but I understand that you are looking for people close by. Did you try googling for 'renewable energy forum' but then in your own language?
In many countries there are also groups of people cooperating to start a solar or wind energy farm together. Perhaps there are also groups near you you can contact?
@THelper Ah, that must be the one, yes. Oh dear, I didn't realise there were no discussion forums for it. And a quick search through my literature database didn't find any researchers in wind active in Romania.