Hi nick87453.
Welcome from me, your fiber optic tree should also run on DC ive had a look inside my misses Mums when it developed a fault, it just a set of lamps and a color filter spins over the lamp. Go for 1000 miliamp adapter or 1 amp, same thing, they normally come with a set of DC plugs, these can be interchanged for size polarity reversal, please read the instructions first, correct polarity may be important, dependent on the circuit in it.
That adapter will power the tree, it really is just a lamp shining on the base of a bunch of fiber optic strands.