where to get Armaco AT-49 transformer?

I'm about to mount 3 pendant lights over an island, I guess my choices are the old-work boxes that have the wings that flip out and hold it in, or the ones with the metal extension bars. The lights are nothing huge, basically a small pole with a globe on the bottom. Would it be OK to use the old work-box or should I go with the other? The old-work box claims that it will hold up to 35 pounds yet the other claims that it will hold only a bit more. Surely these lights won't be anywhere near 35 pounds..... I don't have the lights yet. I figure if I used the old work box I can easily cut the hole and mount it from down below but I want to be sure that it's sturdy. I guess ultimately I'm looking for you guys to say yeh no problem or if I were you I would .... Thanks