Wheel/cassette for indoor trainer

You could just designate a single tire as yoru "winter tire" and then replace it after the season--meaning you'd use it in and out. Of course, that depends on how badly your trainer chews yoru tires. Mine doesn't at all, really, so I rarely even both changing my tire any mroe.

That being said, if you want a cheap wheel and cassette...ebay.

I don't have an extra wheel or tire on mine, I just use my regular ones.  I spend a heck of a lot of time on my trainer - min 2 days a week, max 4 days a week, all winter and summer long.

That said, from a cassette standpoint, keep them the same.  You'll want to have them 'feel' the same, so that if you do stuff like high cadence drills you can have that seem feeling carry over for which gear ratio you find best.  I know there are certain gear ratios that I really love, and others...not so much.

IIWY: It depends on "cheap". Maybe your LBS has a takeoff from someone who upgraded their wheel set time of purchase. I know the one I worked at had a few pairs and they'd sell them for what credit they gave the customer when they bought the bike; usually $150. Nashbar might have one for not too much money.

Also, I'd get / I use an 11-21 cassette. Really, why would you want an "easy" gear while on the trainer?

I use an old tire and burn it up through the winter. I don't see the logic of purchasing a "Trainer tire" when I have 5-6 old ones in the garage from ex-front wheels.

Switching wheels is great b/c you can ride outdoors pretty easily on nice weather days. Changing tires for an outdoor ride is a PIA compared to throwing a different wheel on.