H&M actually carries kids' rain boots in store and they usually have ready stock for toddler sizes, so that might be the quickest option if you need it fast. My neighbour grabbed a pair there for her two-year-old and said the sizing was true to age, though she did mention the range of colours is a bit limited depending on the outlet. Miki House is another option if you want something with better build quality, but the price point is higher and their kids' rain boot stock can be patchy, so it's worth calling ahead to check. Overall, H&M is probably the easier first stop for ready stock without too much fuss, while Miki House suits you more if you're after something sturdier and don't mind the premium.