I think just about every restaurant (at least sit down/buffet) has skim milk (we drink skim). Not as sure about counter service spots. Seems like I've seen skim in single servings at Gasparilla's at the GF, so maybe other resort food courts have it too.
Food Court @ Pop had small (think school lunch) size containers of skim milk when we were there in 2005 and 2006. As I recall they were about $1 maybe $1.29.
Captain Cook's in the Poly had it earlier this month. I don't remember seeing it at the CS places in the parks, but I didn't specifically ask. That said, the only size in the resort was 8 ounce, as others have noted.