Replacement for Lower Case Snap-on brackets?

The 2 plastic brackets that let the lower case snap on or off are broken in my MBP 15" Unibody 2.53GHz Mid 2009 (A1286). Now the lower case bulges out and the MBP won't rest flat on a flat surface.

I looked through the guides and the parts in the store, but I didn't find these parts or instructions on how to fix them.

I'm attaching an image with the broken brackets marked with green circles. (Actually I took the image from the upper case replacement guide, step 23, and added the green circles myself. That is the only picture I found showing both brackets.) The counterparts on the lower case itself seem to be alright - though I'm not sure what they're supposed to look like.

Does anyone know where I can find replacements for these brackets? Did I overlook something in the guides or in the store?

Thanks a lot in advance.