Hello, thank you for purchasing the asset pack!
The flags should all be saved as .png without compression, so if you are seeing blurriness that is most likely being caused by the software you are using to view them. The default Windows image viewer will sometimes blur small images, and so will most game engines by default. You can try to open them with an art software like Paint, Photoshop or Aseprite to check that the raw images are not blurry.
Sometines a filter is applied to pixel art by default when any scaling is happening (Unity and Unreal do this), so you need to set the filter mode to Nearest Neighbour.
If you tell me what software you are using I might be able to help you get the flags to look right!