At the Once Upon a Toy store in Downtown Disney. We just got back home after 12 nights in the world and this was the highlight for my 4 year old Star Wars mad son. After a 8 hour flight from the UK, an hour transfer on the DME to POFQ, a swift check in and look at our room it was off to Downtown Disney for us to build the Lightsabre.
It was great you get to pick either a single or double ended lightsabre with many colours, buttons and handles to choose from, they even give you the batteries!!!
A lovely cast member helped us because it wasn't as easy as it looked to assemble and then when you pay we were asked if we would like a Sith or a Jedi pin. All this for $19.99 + tax, the pin itself is quite large so in my opinion well worth it.