Would also suggest to measure after you’ve given birth. In the first few days, you’ll likely to be latching, which gives you ample time to get the correct flange size.
I got mine from Shopee. If you need the link: https://shopee.sg/product/8051258/851344280?smtt=0.30516520-1653655267.9
Delivery is very fast, I got the flange the next day. :)