For a small job like that, I've had good results with a small magnetic 12 VAC transformers from Radio Shack. There are two models, different amperages, size accordingly. I've actually used them when I made led tape light back lit signs for my concession business and they work fine.
Any Radio Shack should have them.
They are very inexpensive as well. You might just to be extra cautious, put an inline fuse rated for the load capacity of the transformer. Sometimes, when LED lamps fail, ( which has happened on some of my jobs) they produce a direct short. The fuse would protect the transformer from melting up should that happen.