Where can I purchase a replacement drive for this unit?
The 2160A/513 burner (same for both models), PN N78F0FUN, should work OK. It'll come with a Main CBA (PCB) that will have FW for that other model loaded on it, but you can always update your FW to the latest 513 FW or even SuperFW 727V, as described here. (All FW resides on the DVD burner CBA/PCB.)
IMO, best option might be to get the 513 burner, if available, but only install the burner, not the PCB and see how that works first... it *should* work OK. Installing the PCB later, with FW update, is a fallback option.
Others who might need a replacement burner for a 515 be aware that this works seamlessly since he didn't replace the "Main CBA" (PCB) that comes with each new burner. All machine FW resides on that Main CBA, so he retained his 515 FW and all its upgraded operating features.
IF you were to also replace the Main CBA with the 513 CBA, you'd get the FW for the 513, not the 515, so you'd probably want to install SuperFW 727V designed for the 515.