If you can find an extra set of rims for cheap, that is definately the way to go. A website I use on occasion is called partshotlines.com. It does a nationwide search of all the bone yards and lists prices and freight charges.
How can you find out what rims will fit on your vehicle? A simple google search will bring you to websites for car enthusiasts for the model you have. Trust me, it doesn't matter what car you drive, there are people out there customizing them and they will tell you exactly what rims will fit on your car.
When it comes to swapping the rims/tires in the fall and the spring, get yourself a $20.00 floor jack and do it yourself. You can even buy a dedicated socket and wrench for the project. Skiers are hearty people, this is a walk in the park plus you get bragging rights!
As for what snow tires to get? Cross check a number of sites (consumer reports, tire rack, etc.) and see what the general consensus is. If you get the third best tire because you want to save some cash, you will still have a great snow tire. Just get them on all fours.