If you are moving the circuit board and RF chip over you can get shells off ebay. The RF chip looks like a little glass tube and lives in the section with the key blade. If your shells come with a chip it'll probably not work.
If you pop your key apart like you are replacing the battery you will see the part number and FCC ID.
My part number from my 2001 is
4D0 837 231 E 43/00
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