I understand what you're looking for. It's the part on the bottom of the window that the latch catches in to hold the window closed, right? There's two of them on the bottom of each window frame.
This question has come up before, and two things.
1. It's a steel part, not aluminum. (They rust. Badly. Experience talking here
2. No one has replacments for them.
Someone suggested bringing a good one to a sheet metal fabricator and having a new one made, or getting a piece of steel and cutting your own. I'm not sure what the other person ended up doing.