Depending on where you're staying, get the Power Up Band BEFORE you go to the parks - I stayed at Stella Nova and they had them in the gift shop on property. It takes a good 5-10 minutes to open up and set up and figure out the app, so do it ahead of time!
I found that there was different merch in a bunch of different places, so just be willing to explore!
MARIO MOTORS is where you exit the Mario Kart ride and the 1-UP FACTORY is right across on the same ground floor level with the Yoshi ride.
There is a kiosk across from the Yoshi Snack Stand on the way to DK, so check that out too.
Once in DK make sure to check out FUNKY'S FLY AND BUY.
On the way out of the SNW section of the park, check out the SUPERSTAR STORE as well - as i personally think all the best merch is there!
(but i got a yoshi shirt at the kiosk and a dk shirt at Funky's all the same)
Don't forget to check the Nintendo Store at CITY WALK too, if you're going to either of the other parks or that direction for whatever reason.