I just chose 2.75 x 2.75 that way if you wanted to buy your own 8.5 x 11 sticker paper you could print a sheet of 3 x 4 stickers on your own at home if you wanted to. So, yes, you could technically print them on your own if you wanted to.

But as far as companies that do it there are many of them but I have never ordered any custom stickers so I am not sure who offers the best prices, this place for example looks like charges about 26 cents per sticker https://bit.ly/3djbNFa
You could probably also go to like a Fedex/kinkos and give them the file and they could print them for you on the spot (not sure how much they would charge)

If you are cool with this design let me know and I will just put them on my website in a few different sizes.