Do they sell the Disney balloons at downtown Disney the light up ones with Mickey design inside

YES when you exit Disney park. they are on the left side of the area

That's the nearest ones I have seen as well.

Yes

I've actually seen a vendor in Downtown Disney with them there for Christmas last year

Yes left side of the Disneyland exit

yes balloons are sold in Downtown Disney

Yes, they are sold in the large open area at the end of Downtown Disney between the entrance to Disneyland and California Adventure.

I don't recall seeing them there. I could be wrong, but I don't think so