Hi GrannyJen,
Its worth cultivating a relationship with your local tyre shop. I always buy my replacement tyres from them and when I want to switch to my winter wheel set, they do it for free!
(And they are part of a national chain too!)
I store my unused wheels (summer or winter) in the garage but a word of warning - they are filthy things when they are taken off! So grab something like kitchen gloves (or the nitrile gloves that mechanics use) before handling them. Clean them with proper wheel cleaner (brake dust and a damp garage are a great mix for corrosion), dry them and put them away. Canadian friends of mine swear by wheel bags (which you can buy on the 'bay) but I don't bother. The only good thing is you don't have to lift them..you can just roll them!
(Or maybe you can persuade a garage to store them for you - thats a service I understand Porsche offer their clients!).
Enjoy the smug feeling of rolling up and down snowy hills whilst the rest are sliding around!