It was no trouble to find a nice solid knit at Joann Fabrics, however, I couldn't find a blue (too royal!), or a yellow (too orange!) that would suit. I ended up purchasing a blue polo shirt and a yellow polo shirt that were closer to the original colors at a thrift store. They were very inexpensive and I didn't feel wasteful cutting them up for fabric.
I also bought a lighter white knit to be used as lining for each piece. I felt that if I didn't use lining, all my careful cutting would pull and stretch. One of the reasons this dress took so long was that I hand basted each piece of lining on to the dress fabric. I was being meticulous, but now that I'm done, I'm happy with the results. And here they are!