Try your local dollar store. I found disposable plates, cups, silverware, and table cloths in shower colors for my sister's shower at mine. They really weren't any different than the stuff I found at the party stores and were much more affordable.
I second the dollar store...especially for napkins, tablecloths, cups, dessert plates, decorations (although they are usually baby-neutral). I rarely use their plates since they are pretty flimsy. They would be OK for appetizers or cake though.