I haven't lost one, but my dealer told me when we purchased not to lose one as the replacement cost was only a couple of hundred under $1k, which implies they are available but at a price!
Our other vehicle is set up as a remote area 4WD tourer - a big smelly diesel ute. We live in fear of losing its smart key maybe 1500 or 2000km from help. Consequently we keep its spare smart key in the glovebox and have a dumb key wired in a hidden, protected spot underneath. The dumb key can be used to open the vehicle so we can access the smart key to start the vehicle. Where we live we don't consider there is a security issue leaving the smart key in the glovebox, but that could be an issue for you. Anyway, it's an idea that might be useful. If the car gets confused by leaving the fob inside the vehicle you could try putting the fob in a tin box to block its signal.
You can buy replacements although you will need to visit your dealer to get the fob programmed, and the key blank cut, to match your car. Search on Amazon and you should find them. John.
I've got the second fob wrapped in foil and a few other obfuscating layers, and tucked into an obscure nook of the car that isn't typical storage. I made sure that the foil wrap killed the RF link for starting, before committing to that. If you do something like this be sure to swap the fobs once in a while so one key doesn't fall out of hash sequence with the immobilizer or RKE. _H*
My significant other lost the key fob for our Hyundai Elantra and we were able to get a new one from the dealer. With tax, $707 CDN. Ouch! Imagine it would be the same with the Kona. I did look around at cheaper alternatives on the Interwebs, but considering you need the dealer to program it anyway (at a cost) and the potential unknown quality of the cheaper alternative, I bit the bullet and paid Hyundai.
I don't understand why these fobs are so expensive. It is the same for my Mazda. I think it is just a consumer rip-off.