I need a mouse for my iPad 12.9 2nd generation

iPadOS provides support for a basic Bluetooth mouse. Alternatively, you might consider Apple’s Magic Mouse 2:

Magic Mouse 2 - Silver

https://store.apple.com/uk/xc/product/MLA02Z/A

Magic Mouse 2 - Space Grey

https://store.apple.com/uk/xc/product/MRME2Z/A

More information about Mouse and Trackpad support can be found here:

Use a Bluetooth mouse or trackpad with your iPad – Apple Support

I second the Apple Magic Mouse 2, as well.

Best of Luck to You!