You can find them at the little bracelet shop next to the Pioneer Mercantile store in Frontierland (inside Disneyland). They’re sold in the window at the bottom right of the picture above.
First, you buy a bracelet that suits your fancy (there are all kinds of different styles). Then you tell the worker what you want it to be engraved with. In this case, my sister + cousin got matching friendship bracelets and told the worker to engrave “To Infinity” on one, and “& Beyond” on the other. I got one a couple years ago that just has my name on it (boring, but I love it).
Feel free to get creative! The options are limitless.
ETA: If you want to buy them at Walt Disney World, you can find them in the shop outside Pirates.