I'd look at any solid 32 spoke 700c training wheel. Mavic and DT Swiss make good quality spokes and rims. I've heard the entry level 32 spoke wheels at Nashbar are actually pretty bullet proof.

One other thing you want to budget for is a rear cog set for these second wheels. If you are really getting this to quickly swap wheels, then you don't want to have to get your chain whip and cog remover out to swap the cassette. This is relatively time consuming and dirty for a quick wheel change. Otherwise, if you have the time to do this kind of swap, it would be cheaper to just swap the tires.