Where to find pitch/volume button for UR44?

Hello everyone!

I was wondering where I could find pitch/volume buttons for my audio interface UR44?
Here is the screenshot in case it’s not clear: http://glbl.me/cry

The interface travels quite a lot and I got some of them missing now, and without the button, it can be tricky to set up sometimes. Is it possible to buy them in a music shop? Online on Amazon?

Thanks for your help!