After all the work I've done lately, I've realized that I want the undersuit to be pretty darn nice, and I also want any work I do on it to be carry-over onto my Superman Returns project.
For that reason, I just ordered a bunch of black Milliskin from SpandexWorld. This will be the exact same weight and weave as the stuff I'm using on my SR suit, which will come in handy. I also think it's much nicer fabric than the stuff I got at JoAnns.
In general, I've come to the conclusion that the only benefit of going to JoAnns is instant gratification. The fabrics I am buying online tend to be of much higher quality, and are often 40% cheaper. The only bummer is the delay.