I've gotten some on Amazon. I also picked up some at CVS but they were too thick.

If you're going the sticker route, use a peely base coat and don't count on the manicure lasting for very long. I only use them for special occasions or if I'm in a rush but want something fun like for a party. Most of the time I stamp instead. Stamping is just polish so it lasts as well as the rest of the manicure and removes easily.

These KINGMAS stickers are pretty good. https://www.amazon.com/dp/B09MRZP2YH/ I also got this TaliaMei holiday set. https://www.amazon.com/TailaiMei-Halloween-Christmas-Thanksgiving-Decorations/dp/B0B3XNLL7F They're super-cute and mostly fine for my short natural nails but a bit trickier to remove from the backing without tearing or folding them. I use very sharp tweezers.

The reason I don't like the stickers is that even with careful application and a top coat I always have one or two stickers starting to peel around the edges after a day or two. They also block acetone from getting to the polish below so they're even worse to remove than a glitter manicure. The first time I used them I put them over on normal base coat and a couple coats of polish. I ended up having to soak with100% acetone for a long time and then scrape them off the softened polish with an orange stick to dissolve all the polish on my nail. Peely base coat solves the problem.