tried baltic aviation academy?
They offer the type for 757/767 so they should have something.
If that fails check with sabenavita also in lithuania.
There is a charter company here in Vilnius called Aurela, they had the 757 before, don't know though if they have gotten rid off it..
Good luck.
Oh, also check with OAA. But I think that the base is very,very expensive for the 757. Latest figures I heard for the base in Stockholm was 120000SEK...