Search around. Lenovo still sells them, although they are expensive through
that channel, but you know that they'll be legit. I'm not sure what you mean
by how much it would 'cost' to 'install' the WiFi card. I assume that you
would do the installation yourself and that you would not charge yourself for
the act of putting the card in the computer? If you wonder about the cost of
the card itself, they are very cheap - like US$5.
As you search around for WiFi, make note of the part numbers supported by the
ath5k and ath9k kernel modules and make sure that whatever you buy uses a
compatible chipset:
https://wireless.wiki.kernel.org/en/users/drivers/ath5k
https://wireless.wiki.kernel.org/en/users/drivers/ath9k
(Make note of the "Supported Devices" section in ath5k and "Supported
chipsets" in ath9k.
