There are the store bought options, this one is for making your length of pull longer, this one is available from Midway USA
Gun Parts Corp sells these replacement steel ones, they have grooves for traction and come with a rubber spacer
If you have an aluminum one and the anodizing is showing its age, you could strip it and polish it, then replace the original screws with stainless ones (tip: they are a size 12 oval head)
Your original plastic butt plate can be made more stable by stipling its surface with a soldering iron
This owner bought some thin stick on, non-skid tape and trimmed it to fit.
This owner built his own replacement butt plate from thin aluminum diamond plate...
Of course you could trim the back of the stock flat and add a flip flop for a recoil pad, the color choices are almost unlimited and they are cheap
You could also cut your stock, fill in the void on top of the comb and add an aftermarket recoil pad
This guy filled in the crescent area so he could add a flat butt pad
If you are really crafty you could make your self a custom, adjustable butt pad
If you really have your heart set on a factory aluminum butt plate and cannot find one (they are getting rarer by the day), you could always build one, like this guy did