I’ve lost a couple for my car, and was shocked at the price of replacing them from the dealer - over $150 for one!
But there are 3rd party vendors from whom you can buy these things. They teach you how to program it yourself as well.
The one I bought from before was:
(There are other sites out there, but just telling you this was legit and I had success with it)
Using this keylessride.com, I got one for about $40 before a couple of years back.
Since then, I've lost more. Then I tried searching on ebay.com for “keyless” for my car model/make. I lucked out and got two for 29.99. Just make sure you get the programming instructions included.
Programming it is not too hard. It almost seems like those old video game hacks, except it’s w/ your car door. You have to do things like turn on ignition half way, open, then close the door, then lock and unlock 3 times, or something like that.
Anyone should be able to do it.