The part you mention appears similar to a special kind of slotted washer, an example of which I've included in a picture to (hopefully) help in a brief explanation:
In your image the slot looks milled rather than punched as the slot in the image above. If the part is milled it is likely proprietary, meaning you won't find something off the shelf to replace it. Since it would be way cheaper for the makers of your tuner to buy mass-produced slotted washers than to mill their own - it follows that no one is mass producing the size of slotted washer you would need.
That's my guess. If you have a friend who is a machinist or own their own CNC router, you could give them the part and ask them to mill you another, but, lacking this, it would cost more to have the part milled for you, than to buy a new set of tuners.
For the 1MB pot. I don't think G&L makes their own potentiometers, so even if you're concerned about keeping all the parts in your guitar "G&L" sourced, you can drop any long shank 1MB audio tapered pot in, and it'll work fine. You can find them at StewMac or even on Amazon. They are very common and inexpensive enough for you to "splurge" on getting the more expensive ones (which will be almost certainly be a little better than the ones that come standard in the Tribute model, as part of cutting the cost for the tribute involves using less expensive components).
I'd definitely check out StewMac - if for no other reason than the reviews there will be coming from people who bought pots for their guitars. In other words, the reviews will be informed and pertinent to your usage.
Hope that helps.