any grocery store should be able to help.
If you have a SuperWalmart or Target, they could help.
I always do the store made festive holiday ones that are closest to my kid's birthday. Example, we have a brand available in VA called Lofthouse. They have packages of colored holiday cookies that are really good in 12 and 20 packs.
For daughter at daycare:
I always do the Lofthouse cookies or mini cupcakes.
For son at Public school:
Last year I sent in the mini packs of Oreos.
The year before I sent in Little Debbie individual snack cakes. 30 cupcakes tucked into a backpack just like that!
The year before I sent in bookmarks
The year before, mini packs of yogurt pretzels.
For Kinder I did the Lofthouse cookies and juice boxes as that day's snack.