The last two times we visited MK, we had ornaments personalized there. It was $3 per ornament to have my girls' names put on them. They used metallic paint pens, not specifically Sharpies, and while they did ask if I had any color preferences both times I left it up to them and was very pleased.
Leave enough time to have it done, though. The first time we were only in Disney for one day and had to leave by 5:30 PM. Even though we were there by 11AM, pick up time wasn't going to be until 7PM. When I explained why we couldn't have them personalized after all and would just take them with us as-is, the manager promised they'd be done by 5 and they were.