The ONLY place you can get P38 key fobs from is the Land Rover dealer. Yes, they are somewhat expensive, I paid about £170 odd for mine last year - but is came programmed, key blade cut, with batteries and literally just needed syncing to the vehicle.
It does seem like a lot for a 15+ yr old vehicle, but I suppose we should be lucky you can still buy them new! I had a P38 customer come to me for a replacement door latch, and we were having this conversation. He said his wife was wanting a new fiat 500 or something... off the lot, it comes with ONLY one remote key... they asked about a second remote and were quoted over £250 for a second remote... on a brand new car... so it could be worse!
The only thing I'd recommend if you are unsure of your vehicle's history, is to get the VIN number read from the BECM and check to see if it matches what's on the plate under the windscreen. If the BECM was replaced at some point in the past and not reprogrammed, then you might have to order a key for the VIN number of the BECM (as that's what it would be programmed to if not cloned properly!). I only say that because I've heard of a few owners now who have bought a new key for their VIN number, only to find it doesn't work... and then find out the VIN in the BECM is different as it's been replaced, so their new key is useless... (though I can now capture the ID transmitted by a fob, to identify it and check/program it to a BECM - that being said, only one lockset can be programmed to a BECM at any one time, as Rick says.)
As far as I know and have researched, the chips in the fob are OTP - one time program, and they also don't appear to be the UV erasable type either - so once a fob is programmed, that's the ID it is for good.
In theory, and if you could somehow dump the contents of the chip, find the hardware ID in the programming, and then program a NEW chip and swap it onto the fob PCB, then you could effectively make use of an old fob - but to my knowledge this hasn't been possible and done yet. As mentioned, there are some places which can clone an original key, but your replacement is the same ID (say you clone a Key1 - your replacement will be Key1 aswell) - which means that only one can be synced at a time, and they will unsync each other if you try to use both. It's not the end of the world if it's just as a spare key though - but if you want 2 usable keys that are synced at the same time, then it's dealer only.
