There are kv services like xblise, but they're extremely costly. I've never personally used one, but what will happen is your kv will be sent to their (xblise) servers where a fresh kv will be sent to xbox live. This is done through a plugin with dashlaunch (the xbox 360 equivalent of cobra on the ps3). It's not worth it imo. I had heard that they went to being free after people stopped using them due to price. I don't know if that's still true.
Btw, you can also get your cpu key with dashlaunch 3.0+. You just need to open the installer xex file. Dashlaunch is a must when it comes to the 360. You don't technically need it, but it will make things so much easier. With Xell, you either need Xell reloaded or Xellous to transmit video via hdmi; otherwise, u'll get a blank screen when you hit the eject button. Also, changing how to enter Xell is doable with a NAND flash, but I'd stick with the eject button since that's the default option.
And,
@DinohScene is correct. You can extract your kv with a NAND backup. You should definitely ask for one before you buy the console. I've personally kept ones since the xbr days, so I have like ten+ NAND backups for my main jtag.