Try Ebay they have all kinds for sale.

This is off ebay, 25 seeds for about $4.
Red Osier Dogwood (Cornus stolonifera) 25+ Seeds

Zones 2 - 7. Native throughout northern and western North America from Alaska east to Newfoundland and south to Durango. This medium to tall deciduous shrub spreads readity by underground stolens to form dense thickets. Dark red stems make a striking contrast against winter's snow.

The Latin species name, meaning 'bearing stolons", refers to the rooting of branch tips touching the ground and forming new shoots. Since the wood is quite durable, Native Americans used the Red Osier Dogwood as digging sticks, pipe stems, arrows and pegs.

Propagation: Seed propagation requires stratification at 41 degrees F. for 100 to 130 days, so are best to plant directly in the ground in the autumn, However, if not planting until spring, the seeds can be stored in the freezer for a couple of months then start in a greenhouse in small individual peat pots. Plant the seedlings when they have reached four to five inches in height. The plants also root readily from softwood cuttings collected immediately after flowering. Stick cuttings in sand, peat or pearlite for 8 to 10 weeks. Keep moist with mist if possible or high humidity at a minimum. Seed propagation however can work to Red Osier Dogwood�s advantage in keeping populations genetically diverse and therefore less likely to succumb to catastrophic disease or parasite infestations.

Many dogwoods, including C stolonifera grows REALLY easily from cuttings. If you can fine someone with a shrub who will give you some branches you could populate you area much faster by just sticking twigs in the ground. C alba is just as easy.