How can I get a hold of Minecraft's default textures?

The default textures are packaged inside the Minecraft .jar file, so you already have a copy. You can take a look at them with an unarchiver program that understands .jars (really, just zip-format files) such as 7zip, WinZip, etc. You'll find the .jar files in the versions folder of your Minecraft files.¹ If you look inside the .jar file, you'll find all of Minecraft's default textures in the assets folder.

You won't find these files online anywhere legitimate because it's not legal to distribute the files.

1. ^{If you don't know where your Minecraft files are located, the easiest way is to start Minecraft, click Options then Resource Packs, then click Open resource pack folder. That will tell your OS to open the resourcepack folder. From there, go up one folder and you'll be looking at your Minecraft install, and versions will be one of the folders there.}

On a windows PC, hold 'Win+R' and type %appdata% This should automatically go to Roaming, but if not simply go to the roaming folder. Navigate to the .minecraft directory, and go to the 'Versions' Folder. Find the version of minecraft you are trying to make a texture pack for, and open it.

Create a Folder on your desktop, and copy the .jar and .json files into it. Make sure you copy. If you do not have a program like Win-Rar or 7-Zip, install one now. Right Click on the .jar and select 'Open With Win-Rar' or whatever your archiver may be. Delete META-INF, and go into the 'assets' folder. Change Whatever textures you like.

Once Finished, make a new folder in the versions folder called '1.x_resource1' or something. Paste the .jar and .json from your desktop into the new version, and select it from your launcher.

You can download textures from multiple versions from the mcasset.cloud site, which is not affiliated with Mojang.