Is there a way to download the default Minecraft 1.17 textures?
You can extract the minecraft textures from the minecraft JAR file, you should be able to find the latest 1.17 snapshot JAR file downloads on the Minecraft Wiki.
You can then use an archive utility (like 7-zip) to extract the JAR and edit the textures (which are located in the assets/minecraft/textures folder), then just copy the folder to the place you want to make your texture pack.