Yes, I'm sure that you can find a place in Tallinn where you can buy a bubble blower. And I think bringing the bubble blower to the venue is not a problem, because back in 2010 they let us in with the bubble blowers. And I'm not quite sure about it yet, but there should be a place on the festival grounds where they are going to sell bubble blowers before the gig.
And if you are coming to the venue from the city center, then you can get there easily with buses number 1A, 8, 5, 34A and 38. I think there should be information about how to get there on the homepage of the venue as well: http://lauluvaljak.ee/en/
I hope it helped