Register with a seed exchange. It will cost somewhere between $10 and $100, depending on the site you select. While you can buy or exchange seeds without registering, you will have better access to other gardeners and a wider selection of seeds available, which will make finding blackberry seeds easier. Wild Oregon blackberry seeds may be hard to find; be prepared to pay $20 to $50 for 10 to 20 seeds.
Post requests for wild Oregon blackberry seeds in gardening chat rooms, blogs or websites.
List the heirloom or rare seeds you have to offer, if any. If you have cuttings of easily rooted perennials or shrubs list those as well. By offering your own seeds and cuttings, you may encourage someone else to gather wild blackberry seeds for you in exchange for something you have. If you don't have your own seeds or cuttings, check with gardeners in your area; most gardeners are happy to share seeds (or cuttings) from common plants like zinnias, cosmos and most vegetables.