Some merchandise is available in all parks during all times of the year. Other merchandise is not. If you visit often and see something unique, scoop it up! Don’t wait for the next visit or wait till the end of your trip, because it might be gone!
Don’t worry though, if you have to have something, likelyhood is that you will pay a premium for it.
Example, My little one wanted a full “tea set” of the Starbucks ornament mugs. I waited for a visit when she wouldn’t see me purchase them and poof – Magic Kingdom and Epcot were sold out! I paid more than twice on eBay for these two items to complete her collection!