The place for this request is really on one of the exchange forums, either a regional one (good for location appropriate donations) or the general or plant specific exchanges. While these are typically trading exchanges, some folks are very generous or have large amounts of seeds/plants they are willing to share for just shipping expense.

Other sources of inexpensive plants are friends and neighbors who would be willing to divide and share some of their existing plants. You may also want to learn how to propagate specific plants by cuttings. It is easy to find instructions online and there is a forum here on GW that focuses on plant propagation that can answer questions. I'd also look at Master Gardener plant sales in your area for inexpensive purchases. Church groups often have plant sales as fund raisers as well.

It is important to remember that gardens are created/formed over time, not over night. Be patient and go about it slowly. And take some time early on to research what types of butterflies are common to your locale and what plants they favor, both as nectar sources and as nesting habitat/food stock for their offspring. That way you will know exactly what kind of plants to be looking for.

1) join a local garden club, people share their extras

2) watch out for sales, usually towards end of season which is good as it provides plants for NEXT season

3) keep in mind that online requests usually require that you pay for shipping though plants/bulbs/seeds are free. May be cheaper to buy local plus local plants usually mean they are appropriate for your area.

Why not go to the seed exchange here and sign up for a newbie donations pack?

Ditto to all above. Add to that local seed exchanges. With just a little work and diligence on your part, you can find many sources of cheap, even free, plants.