Both are abandonware, so you can get them free. There's also some open-source projects following X-com, best one is UFO:Alien Invasion,
http://ufoai.ninex.info/ . Unfortunately that's still not a complete game, but looks great so far. Last time I played it it got boring after a couple of missions since there's a lot of balancing and cutting of redundant stuff that still needs to be done.
A problem with the original is that X-Com has a bug so you can't play on harder difficulties. This never got patched. So once you've played it through there's no extra challenge, unless you want to play it iron-man, without saves. Terror from the Deep was a bad sequel, didn't really bring much to the game. The original X-com was better in all respects.