Just about any cheap wheel will do for trainer use.
Check your local Craig's List, peruse internet auction sites, and don't forget your local bike store - you may find they have "take offs", or wheels removed from new bikes they sold. Lots of stronger/heavier riders can't really ride some of the lightly-built 24- or 28-spoke rear wheels that come on some road bikes, so they'll buy a bike and a stronger set of wheels, leaving the bike store with a set of lighter wheels to sell.
But even if you're larger/heavier, riding a trainer isn't all that rough on a rear wheel - the wheel isn't supporting your weight, nor is it being subjected to impacts from hitting things while supporting your weight. And even if you're really strong, suddenly hammering the pedals in a sprint effort doesn't put anywhere near the stress on the wheel that doing the same outside does. On a trainer, if you spike your power really hard during a sprint effort, you'll just cause the tire to skip on the trainer's roller instead of passing a huge torque spike from your effort through the wheel to accelerate your and your bike.
One thing that does matter more on a trainer - trueness. You can actually get away with a wheel being quite a bit out-of-true outside, as the tire will absorb a lot. Not so on a trainer, especially if you have any in-and-out or "hop" in your wheel. You want a nice even engagement between the tire and the trainer's roller.
And if you're not using a "smart" trainer that can vary resistance to do things like simulate a real climb - don't use a wide-range cassette on a trainer - no 11-32s. Get something with a closer range. For a 9-speed, I wouldn't want anything much wider than an 11-25 or so, and ideally an 11-23 corncob. The close ranges are necessary so you can closely tune your cadence, power, and effort level. For example, if you want to do a 90-min ride at, say, 90 RPM, you may find a wide-range cassette limiting your options to 81 or 97 RPM at an effort level you can sustain for 90 min. You're not likely to even be able to use a 32 or even a 28 on the trainer, but there are uses for the 11 - simulating steep hill climbs, for example.