I actually found that they were pretty well stocked on the 4/28 cruise. I tried to go between the show and dinner, which was a terrible idea as everyone had the same thought and it was super packed. I didn't have issues with not finding sizes, etc. I went back on day 3 (which was supposed to be
Castaway Cay, but instead was a sea day due to mechanical issues). I actually really liked that we had a sea day because it was more time the shops were open (especially since they ended up being closed pretty much all day on day 2 since we were stuck in port). Crowds were very minimal that day, and even though it was the last full day of the cruise I had no issues with sizes.
Dory's shop up on 11 (?) outside Marceline Market did have inventory issues, though. I grabbed the white
DCL rash guard spirit jersey earlier on day 3, then realized I needed to exchange the size. I went back between the show and dinner; the shop was empty, but the cast member told me they were low on sizes of that (luckily they had what I wanted to exchange to).
If there's something you reallllly want, I'd say fight the crowds and go as soon as you can (probably between the show/dinner on the first night), just for peace of mind that you have it. Yes it was crowded when I did that but it wasn't too awful. If you just want to look around, then waiting until night 2 or 3 would probably work.