You might consider a tall grass. I have an unidentified tall miscanthus that grows amongst the roots of a large fir tree in unmodified soil. It is very vigorous. I moved it from a mostly sandy full sun area after it outgrew that bed to this dry partially shaded clay soil area and it has doubled in size in a few years. The one I have needs a part cut off each spring to keep the clump to the allotted space but there are other varieties of grasses that grow much slower. That's probably why it was shared at a spring swap where I picked it up. It is a nice grass with beautiful tassels. Grasses can be started from seed. Few seed displays offer grass seed but there are many online sources. Rudbeckia and echinacea are good companion plants for grasses and can also be started from seed.

Check your area for plant swaps (some don't require you to bring plants the first year) and also check your area for city yard waste facilities that let residents take free compost.

Start conversations with neighbors or others about their gardens. After they get to know a person is interested in gardening gardeners are very generous sharing divisions of their plants without being asked.

If you need to buy a large quantity of compost/soil buying in bulk from a nursery is less expensive than buying individual bags and usually better quality.

These inexpensive ways have all helped me expand my gardens.