Plain paper envelopes for seed.

For covering flowers to prevent stray pollen, anything that lets air pass and is light-colored to minimize heat build up will do.

Many people use the organza bags sold for holding wedding favors - cheap and easy to find.

Thanks for replying.. But I'm kind of looking more for people that have actually bagged their plants to prevent cross-pollination, and what bags they have used to do so.

The organza bags mentioned above are great. The fabric is very sheer but tightly woven, the draw strings make it so easy, and they do come in many sizes. If you are doing something like peppers though, it is often easier to drape the whole plants with a sheer light weight fabric.
I've never used a a dark type fabric.