Where does one purchase that quantity of sunflower seeds and how much would it cost to do a few acres (3 or 4)?
You can buy oilseed sunflower seed, sold as bird seed in most stores. One of the ag stores near here will sell you as many pounds as you want. I just get the birdseed variety, germination was good for me. The deer liked them too much, didn't get to see many flowers. Mark
Cut and paste from University of Missouri Extension.
Planting
The most critical planting requirements are getting the seed into adequate soil moisture and choosing an appropriate seeding rate. Planting date and row widths can vary considerably without seriously affecting production. For heavy soils, seeding depth should be 1 to 1.5 inches, depending on depth to moisture.
Seeding rate is especially important for sunflower varieties intended for the snack food market. A high seeding rate will cause seed to be smaller, which is undesirable for snack food. A general recommendation for this type of sunflower is 17,000 seeds per acre (roughly 4 pounds per acre), while a general recommendation rate for oilseed varieties is 23,000 seeds per acre (roughly 3 pounds per acre). The higher seeding rate is recommended for oilseed types to improve yields. Seed size will be reduced, but market prices for oilseed sunflower are unaffected by seed size.
Yields from April or May plantings normally will be higher than June or July plantings. Sunflower can germinate and emerge from soils as cool as 46 degrees Fahrenheit if the seed is of good quality. Frost will not cause long-term damage until plants are around the six-leaf stage, which is normally after the danger of frost has passed. Mature sunflower also tolerates low temperatures, being able to withstand temperatures down to 25 degrees Fahrenheit.
Row spacings should be wide enough to permit cultivation, typically 20 to 30 inches. Various row spacings show little effect on yield, except that dwarf sunflower will close a canopy better and yield more when rows are less than 30 inches apart. Row orientation is another factor often debated for sunflower. Research has shown no yield difference among possible row orientations, but some have suggested that a north-south row orientation can make harvesting easier.
A common planting method is to equip a corn planter with special sunflower seed plates, which are sold by various companies. A fungicide seed treatment prior to planting is often recommended.