For external drives, pretty much any drive out there should work as long as it's formatted in a way the iPad can read.
exFat, HFS+ or APFS.
You'll also need a drive that has a USB-C connection or an adapter to plug it into the iPad's charging port.
There are many USB-C drives out there that will work directly with an iPad but you may need to format them yourself as they probably will come formatted for Windows.
For example click here ➜ Sandisk 1TB USB-C SSD - Amazon.com
2 things to note,
- You will not be able access the photos on the external drive from the iPad's Photos app directly. Access will be through the Files App. If you want to view and organize them in the Photos app, you will need to import them.
- click here ➜ Import and export photos and videos on iPad - Apple Support
- To Import the Photos, they must adhere to certain requirements.
- Be located in a folder labeled DCIM
- Have a file name 8 characters in length with a 3 character extension. For example: img00023.jpg
More info here: click here ➜ Connect external storage devices to iPad - Apple Support