Always get a spare key or two cut BEFORE you need them! Do it today.

Have one set of spares cable tied (zip tied?) somewhere on to your wheelchair or even on to your van (hidden underneath) or somewhere for use in an emergency. 

Most high streets or markets will have a guy that copies keys. Even vehicle keys. If your key is a modern complex thing with radio controlled door locking etc then you may need to get that from your main dealer and they are not cheap. But MUCH cheaper than being rescued because you lost your keys!

A wheelchair is a great place since a wheelchair user will not be getting far without his wheelchair! So you will ALWAYS have your spare keys on you. 

And have one also in a safe known place in your home that others have access to in case all else fails. Also while you are at it get copies of all your important keys (house etc) and keep them all together with you on your wheelchair or powerchair.

To keep fancy expensive vehicle keys with electronics and built in transmitters safe from water just get a tough plastic bag and add your key(s) to it and tie a knot!

Then attach this under your seat or somewhere in your powerchair or wheelchair. One day you will be very pleased you went to the bother and expense of doing this!

If you are really good at losing keys there are various "key insurance schemes" available. These are not expensive and cover the cost of changing locks, getting new keys and being "rescued" so to speak.

But surely the cost of having a few spares cut and putting them in safe and useful positions is a better solution since you don't need to be "rescued" and suffer no waiting, cold, inconvenience or hardship.