can i get weed in tallinn?

This question shows the reasons to hold stag does in your own town, you know where to get everything and you know the risks you run.

Yes, weed is being sold in Tallinn, it is being sold in almost every town in the world.
Yes it is illegal too, and it is a silly thing to do something illegal in a foreign country where you do not know the risks of doing illegal things nor speak the language to deal with the results. (You can even try to buy weed from a police officer and not understand the warnings he gives you, as you do not speak the local speech.)

Drinking abroad is also a bad idea, if only that you spread the poor name of your country even further.

Added:
The first answerer is wrong, any kind of drugs are illegal in the Netherlands, some kinds are not acted against if you make no problems.
But traveling while high or with any kind of drugs is a bad idea too. Very strict checks of your luggage is the least you can expect when they think you have visited a coffee shop to buy on your way somewhere else, a full body search, (undress and all body openings searched) is really likely.